Use golangci-lint fmt to format code (#163)

Use `golangci-lint fmt` to format code, upgrading `.golangci.yml` to v2 and mirroring the linter configuration used by https://github.com/go-gitea/gitea. `gci` now handles import ordering into standard, project-local, blank, and default groups.

Mirrors https://github.com/go-gitea/gitea/pull/37194.

Changes:
- Upgrade `.golangci.yml` to v2 format with the same linter set as gitea (minus `prealloc`, `unparam`, `testifylint`, `nilnil` which produced too many pre-existing issues)
- Add path-based exclusions (`bodyclose`, `gosec` in tests; `gosec:G115`/`G117` globally)
- Run lint via `make lint-go` in CI instead of `golangci/golangci-lint-action`, matching the pattern used by other Gitea repos
- Apply safe auto-fixes (`modernize`, `perfsprint`, `usetesting`, etc.)
- Add explanations to existing `//nolint` directives
- Remove dead code (unused `newRemoteReusableWorkflow` and `networkName`), duplicate imports, and shadowed `max` builtins
- Replace deprecated `docker/distribution/reference` with `distribution/reference`
- Fix `Deprecated:` comment casing and simplify nil/len checks

---
This PR was written with the help of Claude Opus 4.7

Reviewed-on: https://gitea.com/gitea/act/pulls/163
Reviewed-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: silverwind <me@silverwind.io>
Co-committed-by: silverwind <me@silverwind.io>

pkg/artifactcache/handler.go

@@ -15,12 +15,12 @@ import (
 	"sync/atomic"
 	"time"
 
+	"github.com/nektos/act/pkg/common"
+
 	"github.com/julienschmidt/httprouter"
 	"github.com/sirupsen/logrus"
 	"github.com/timshannon/bolthold"
 	"go.etcd.io/bbolt"
-
-	"github.com/nektos/act/pkg/common"
 )
 
 const (
@@ -74,7 +74,7 @@ func StartHandler(dir, outboundIP string, port uint16, logger logrus.FieldLogger
 	if outboundIP != "" {
 		h.outboundIP = outboundIP
 	} else if ip := common.GetOutboundIP(); ip == nil {
-		return nil, fmt.Errorf("unable to determine outbound IP address")
+		return nil, errors.New("unable to determine outbound IP address")
 	} else {
 		h.outboundIP = ip.String()
 	}
@@ -363,7 +363,7 @@ func findCache(db *bolthold.Store, keys []string, version string) (*Cache, error
 			}
 			return cache, nil
 		}
-		prefixPattern := fmt.Sprintf("^%s", regexp.QuoteMeta(prefix))
+		prefixPattern := "^" + regexp.QuoteMeta(prefix)
 		re, err := regexp.Compile(prefixPattern)
 		if err != nil {
 			continue
@@ -415,6 +415,7 @@ const (
 	keepOld    = 5 * time.Minute
 )
 
+//nolint:gocyclo // function handles many cases
 func (h *Handler) gcCache() {
 	if h.gcing.Load() {
 		return