Fixes issue #33231 and makes it possible to enable Plasma and KDE at the same time.

Previously, this worked like this:
- The gdk-pixbuf package comes with a cache file covering the modules bundled
  with gdk-pixbuf.
- The librsvg package comes with a cache covering modules from gdk-pixbuf as
  well as librsvg.
- plasma5 and xfce modules set the environment variable GDK_PIXBUF_MODULE_FILE
  to the one from librsvg, so that SVG was supported in addition to the
  formats supported by gdk-pixbuf. However if both were enabled a configuration
  conflict would result (despite setting to the same value).

While this sort of worked (ignoring the conflict which perhaps could be hacked
around), it is unscalable and a hack, as there would be a real problem when one
wanted to add a third package that supports additional image formats.

A new NixOS module (gdk-pixbuf) is added with a configuration option
(modulePackages) that other modules use to request specific packages to be
included in the loaders cache. When any package is present in the list, the
module generates a system-wide loaders cache which includes the requested
packages (and always gdk-pixbuf itself), and sets the environment variable
GDK_PIXBUF_MODULE_FILE to point to the generated cache file.

The plasma5 and xfce modules are updated to add librsvg to modulePackages
instead of setting GDK_PIXBUF_MODULE_FILE.

Note that many packages create wrappers that set GDK_PIXBUF_MODULE_FILE,
some directly to the one from librsvg. Therefore this change does not
change the existing hack in the librsvg package which ensures that
file is generated. This change aims only to solve the conflict in the
global environent variable configuration.

This fixes a regression introduced in commit
  700e21d

nix needs ssh on path for the SSH substituter functionality,
not only the distributed builds functionality.

Signed-off-by: Niklas Hambüchen <mail@nh2.me>

Semi-automatic update generated by
https://github.com/ryantm/nixpkgs-update tools. This update was made
based on information from
https://repology.org/metapackage/evdi/versions

When a bridge interface was reconfigured, running containers using
this bridge lost connectivity: restarting network-addresses-brN.service
triggered a restart of network-setup.service via a "partOf" relationship
introduced in 07e0c0e.
This in turn restarted brN-netdev.service.
The bridge was thus destroyed and recreated with the same name but a new
interface id, causing attached veth interfaces to lose their connection.

This change removes the "partOf" relationship between
network-setup.service and network-addresses-brN.service for all bridges.

no reason to make this part conditional

This is more clear, to separate test dependencies.

Semi-automatic update generated by
https://github.com/ryantm/nixpkgs-update tools. This update was made
based on information from
https://repology.org/metapackage/harfbuzz/versions

Semi-automatic update generated by
https://github.com/ryantm/nixpkgs-update tools. This update was made
based on information from
https://repology.org/metapackage/unbound/versions

Old git source repository was dead. Switched to GitHub

Semi-automatic update generated by
https://github.com/ryantm/nixpkgs-update tools. This update was made
based on information from
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From the changelog:

- The standard library's `str::repeat` function contained an out of bounds write
  caused by an integer overflow. This has been fixed by deterministically
  panicking when an overflow happens.

… to lib.

rustc: 1.29.0 -> 1.29.1

Semi-automatic update generated by
https://github.com/ryantm/nixpkgs-update tools. This update was made
based on information from
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remove support for pre-5.26 Perl

remove support for pre-5.26 Perl (2)

Semi-automatic update generated by
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based on information from
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help2man: 1.47.6 -> 1.47.7

…iptions

Several service definitions used `mkEnableOption` with text starting
with "Whether to", which produced funny option descriptions like
"Whether to enable Whether to run the rspamd daemon..".

This commit corrects this, and adds short descriptions of services
to affected service definitions.

musl support was added upstream in v2.0.12 in
numactl/numactl@31dc295

Slurm looks for lz4 by default, and if it finds it can improve its compression techniques

mesa: 18.2.1 -> 18.2.2

changelog: https://github.com/hashicorp/vault/blob/v0.11.2/CHANGELOG.md

Presently, gnupg1compat only works with gnupg22. Without this change, the error

```
error: attribute 'version' missing, at .../nixpkgs/pkgs/tools/security/gnupg/1compat.nix:4:26
```

is emitted when evaluating

```
pkgs.gnupg1compat.override { gnupg = pkgs.gnupg20; }'
```

E.g. exa was wrongly put into /lib, as it matches

  .*.a

but not

  .*\.a