build: cover Windows Go files (#1052)

- fix Windows-only golangci-lint findings in `lp_windows.go` and process killer handle cleanup
- add a `lint-go-windows` target that runs golangci-lint with `GOOS=windows`
- include the Windows lint pass in `make lint` so Ubuntu CI covers `_windows.go` files

Reviewed-on: https://gitea.com/gitea/runner/pulls/1052
Reviewed-by: Zettat123 <39446+zettat123@noreply.gitea.com>

act/lookpath/lp_windows.go

@@ -62,7 +62,7 @@ func LookPath2(file string, lenv Env) (string, error) {
 	var exts []string
 	x := lenv.Getenv(`PATHEXT`)
 	if x != "" {
-		for _, e := range strings.Split(strings.ToLower(x), `;`) {
+		for e := range strings.SplitSeq(strings.ToLower(x), `;`) {
 			if e == "" {
 				continue
 			}