ci: run nix build in workflow

so since up until the last merged changes, this entire gh action pretty
much equated to a no-op, afaict. and even now, the only thing i added in
PR #272 is a formatter check only for *.nix files.

i plan to add more checks soon that will be run by `nix flake check`.
but as of right now, adding this is the quickest way to make sure at
least something useful is done by this workflow.

Signed-off-by: Thomas Mühlbacher <tmuehlbacher@posteo.net>

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