Defer musl iserv source selection through the module fixed point

.github/workflows/nix.yml

@@ -96,7 +96,9 @@ jobs:
       - name: Check musl interpreter link policy scope
         if: ${{ matrix.target == 'linux-static' }}
         timeout-minutes: 1
-        run: nix eval --json --file scripts/nix/check-musl-iserv-scope.nix
+        run: |
+          nix eval --impure --json --file scripts/nix/check-musl-iserv-scope.nix \
+            --apply 'f: f { lib = (builtins.getFlake (toString ./.)).inputs.nixpkgs.lib; }'
 
       - name: Check Intel Setup failure selection
         if: ${{ matrix.system == 'x86_64-darwin' && matrix.target == 'all' }}