Resolve project root to find secrets.nix

[mweinelt]

It would be very ergonomic if agenix wouldn't have to be executed from within the directory containing the secrets.nix, but instead resolved its path somewhere below the project root, possibly using git rev-parse --show-toplevel.

$ agenix -e secrets/alertmanager-secrets.age
error:
       … while calling the 'concatStringsSep' builtin

         at «string»:1:39:

            1| (let rules = import ./secrets.nix; in builtins.concatStringsSep "
             |                                       ^
            2| n" rules."secrets/alertmanager-secrets.age".publicKeys)

       … while calling the 'import' builtin

         at «string»:1:14:

            1| (let rules = import ./secrets.nix; in builtins.concatStringsSep "
             |              ^
            2| n" rules."secrets/alertmanager-secrets.age".publicKeys)

       error: getting status of '/home/hexa/git/mrmcd/nixos-config/secrets.nix': No such file or directory