workflows/check-nix-format: show command to fix

When some files are not formatted properly, this shows how people can
fix the problem.

This notably uses the shell.nix introduced in the parent commit to
ensure that the nixfmt version matches what CI expects.

.github/workflows/check-nix-format.yml

@@ -56,6 +56,7 @@ jobs:
           NIX_FMT_PATHS_BUILD_SUPPORT_PHP: pkgs/build-support/php
         # Iterate over all environment variables beginning with NIX_FMT_PATHS_.
         run: |
+          unformattedPaths=()
           for env_var in "${!NIX_FMT_PATHS_@}"; do
             readarray -t paths <<< "${!env_var}"
             if [[ "${paths[*]}" == "" ]]; then
@@ -64,7 +65,12 @@ jobs:
             fi
             echo "Checking paths: ${paths[@]}"
             if ! nixfmt --check "${paths[@]}"; then
-              echo "Error: nixfmt failed."
-              exit 1
+              unformattedPaths+=("${paths[@]}")
             fi
           done
+          if (( "${#unformattedPaths[@]}" > 0 )); then
+            echo "Some required Nix files are not properly formatted"
+            echo "Please run the following in \`nix-shell\`:"
+            echo "nixfmt ${unformattedPaths[*]@Q}"
+            exit 1
+          fi