stylix: make nix-flake-check package execution location-independent

Make the execution of the nix-flake-check package independent of the
current working directory, making the following command work as
expected:

    nix run github:danth/stylix#nix-flake-check

This is especially convenient for testing PRs locally without checking
them out.

flake.nix

@@ -168,7 +168,10 @@
                       --color-failed \
                       --halt now,fail=1 \
                       --tagstring '{}' \
-                      'nix build --no-update-lock-file --print-build-logs .#{}'
+                      '
+                        nix build --no-update-lock-file --print-build-logs \
+                          ${self}#{}
+                      '
                 '';
               };