Since using flakes disallows the usage of <unstable> (which I use in
some tests), this adds an alternative. By setting specialArgs, all VMs
can get the `unstable` flake input as an arg. This is not possible with
extraConfigurations, as that would lead to infinite recursions.

nixos/testing: Add support for specialArgs

fac-build: init at 0.5.3

Curl is a new requirement so the weather plugin can use libcurl to pull
weather data.

IPython versions 7.17 and higher don't support Python 3.6 anymore, so
this commit adds back the old expression for 7.16.1 from before
ba1038a for use in python36Packages
only.

    nix-repl> :b python36.withPackages (ps: [ ps.ipython ])
    builder for '/nix/store/q8v4f89xwv35a3idb9z345z6n3nzfycb-python3.6-ipython-7.17.0.drv' failed with exit code 1; last 10 log lines:
      Python 3.5 was supported with IPython 7.0 to 7.9.
      Python 3.6 was supported with IPython up to 7.16.

      See IPython `README.rst` file for more information:

          https://github.com/ipython/ipython/blob/master/README.rst

      Python sys.version_info(major=3, minor=6, micro=12, releaselevel='final', serial=0) detected.
    cannot build derivation '/nix/store/mhpdarp18z6skzswrl7sbgzv8hr4gwih-python3-3.6.12-env.drv': 1 dependencies couldn't be built
    [0 built (1 failed)]
    error: build of '/nix/store/mhpdarp18z6skzswrl7sbgzv8hr4gwih-python3-3.6.12-env.drv' failed

Incompatible with astropy>=4.0

```
  Processing ./hickle-4.0.1-py2-none-any.whl
  ERROR: Package 'hickle' requires a different Python: 2.7.18 not in '>=3.5'
```

mbtileserver: init at 0.6.1

omnisharp-roslyn: 1.35.2 -> 1.37.1