ci/parse: init

The nix-parse workflow can now be run locally the same way as in CI.

To do this, the CI's workflow was slightly adjusted. Instead of testing
only the changed files, we're now testing all files in the repository.

This is possible in two ways:

1. By calling nix-instantiate once with all files as arguments. This
will be rather fast, but only the first error is shown before it errors
out.
2. By calling nix-instantiate once for each file. This will be much
slower, but has the advantage that we see all errors at once.

To avoid running the long variant every time, we first do a quick check
with the fast version. If that fails, we run the slower one to report
the errors. This gives us the best of both.

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