chore: scope fmt-check to Go sources (#1125)

`fmt-check` runs `make fmt` and then diffs the whole working tree, so any unrelated uncommitted file makes it fail with `Please run 'make fmt' and commit the result` even when the formatter changed nothing.

Restrict the diff to the files the formatter can touch, matching what [gitea's Makefile](https://github.com/go-gitea/gitea/blob/main/Makefile) does.

Reviewed-on: https://gitea.com/gitea/runner/pulls/1125
Reviewed-by: techknowlogick <[email protected]>
Co-authored-by: silverwind <[email protected]>

Makefile

@@ -97,7 +97,7 @@ go-check:
 
 .PHONY: fmt-check
 fmt-check: fmt
-	@diff=$$(git diff --color=always); \
+	@diff=$$(git diff --color=always -- '*.go'); \
 	if [ -n "$$diff" ]; then \
 		echo "Please run 'make fmt' and commit the result:"; \
 		printf "%s" "$${diff}"; \