Update nix lock file and check in CI that nix lock file is in sync with nix flake (#4838)

Recently, the nix flake was updated in a pull request without fully
updating the respective nix lock file. This causes issues when trying to
install a nix derivation from CLI, i.e. with `nix` or `kup`, without
cloning the repository, as the checked out source is placed in the
read-only nix store. Therefore, the nix lock file must always stay
perfectly in sync with the nix flake.

This pull request updates the nix lock file to be in sync again.
Additionally, the flag `--no-update-lock-file` was added to CI when
building the nix derivation to check that the nix lock file is actually
in sync. Otherwise, CI would silently update the nix lockfile without
reflecting that change in a commit. This additional sanity check could
also be added to other RV repositories with CI that use nix.

Author: juliankuners

.github/workflows/test-pr.yml

@@ -219,7 +219,7 @@ jobs:
           nix --version
           JQ=$(nix-build '<nixpkgs>' -A jq --no-link)/bin/jq
           export JQ
-          k=$(nix build . --print-build-logs --json | $JQ -r '.[].outputs | to_entries[].value')
+          k=$(nix build . --print-build-logs --json --no-update-lock-file | $JQ -r '.[].outputs | to_entries[].value')
           drv=$(nix-store --query --deriver "$k")
           nix-store --query --requisites "$drv" | cachix push k-framework
       - name: 'Smoke test K'