Flatpak’s installed tests build Flatpak runtimes, among other things.
Upstream code does this by copying some programs on `PATH`
as well as some possible dependencies from host’s /usr.
We patch the code to use `nix-store --query --requisites`
to make the dependency discovery easier.

The Flatpak’s installed tests add `socat` to `PATH` and later run
`nix-store --query --requisites` on its location but it was failing with

    error: path '/nix/store/qcyf7nq5vvfw32967sv4j6z190inrbrc-socat-1.7.3.4' is not valid

The issue occurred because, while the host Nix store is bind mounted into the test VM,
the VM’s store uses its own database that only contains the packages in the VM’s closure.
Since the test commands are not actually part of the VM but only passed through PTY,
the `flatpak.installedTests` derivation was not part of the VM’s closure, so `nix-store`
in the VM could not get information about its dependency `socat`.

Let’s make the `installedTests` of the tested package part of the test VM’s closure
by passing it as a global environment variable. This will also have the added benefit
that user no longer has to type the path when running the installed tests manually in the VM;
they can just use `gnome-desktop-testing-runner -d $TESTED_PACKAGE_INSTALLED_TESTS`,
which is much more conducive to tab completion.

Along with the `socat` fix in the parent commit, this makes
the Flatpak’s installed tests finally pass again.

The tests seem to need slightly over 2G of disk space,
and it appears that the test suite was ported to Python 3 in 1.5.1:
flatpak/flatpak@2b66415

Changes:
* https://github.com/flatpak/flatpak/releases/tag/1.7.1
* https://github.com/flatpak/flatpak/releases/tag/1.7.2
* https://github.com/flatpak/flatpak/releases/tag/1.7.3
* https://github.com/flatpak/flatpak/releases/tag/1.8.0
* https://github.com/flatpak/flatpak/releases/tag/1.8.1

Commits:
flatpak/flatpak@1.6.3...1.7.1
flatpak/flatpak@1.7.1...1.8.1

Notable packaging changes:
* Flatpak now ships a sysusers.d file for allowing systemd to create the required users.
  flatpak/flatpak@4df0190
* Completion support for fish shell
* If an app has filesystem access, the host /lib is accessible as /run/host/lib, etc.
* New filesystem permission "host-etc" and "host-os" give access to system /usr and /etc.
  flatpak/flatpak@fe2536b
* We now always expose the host timezone data, allowing us the expose the host /etc/localtime in a way that works better, fixing several apps that had timezone issues.
  flatpak/flatpak@dc4e198
* We now ship a systemd unit (not installed by default) to automatically detect plugged in usb sticks with sideload repos.
* By default we no longer install the gdm env.d file, as the systemd generators work better
  flatpak/flatpak@7c3a85b
* Use variant-schema-compiler for some GVariant code
  flatpak/flatpak#3366
* zstd compression for oci deltas:
  flatpak/flatpak@bfa71e2

Additionally:
* Remove glibcLocales which is not used since 1.4 bump because glibc contains a locale archive with C.UTF-8
  1728bc8
* Stop using aliases for docbook-xsl-nons and pkg-config packages
* Stop using autoreconfHook, the autogen.sh script contains some extra that are necessary when building from git.
* Increase disk space for installed tests, they were running out.
* Enable building developer documentation.

GNOME is not necessary. Portals probably are not either,
but the NixOS module requires them.

Not sure why it did not work without GNOME before.