fix: stop artifact uploads depending on the cache server reaching Gitea (#1216)

Cache v2 makes the cache server the `ACTIONS_RESULTS_URL` origin, so artifact calls arrived there and were proxied on to Gitea, failing whenever it could not reach the instance.

- Artifact calls are answered with a redirect, so the cache server opens no connection to Gitea. A scheme change or an untrusted instance is still proxied, but there the cache server is the runner itself, which already reaches Gitea.
- Failures answer in twirp, not an empty `502` that clients report as `Unexpected end of JSON input`.
- `cache.v2: false` really points artifacts at Gitea now.
- Cache reservations are bound to the job that made them, so two jobs saving one key cannot commit against each other's upload, and a retry after a lost answer no longer fails a saved entry.
- The toolkit patch, which edits the GitHub-host check out of an action's bundle, was left in the shared checkout where a job running with `runner.patch_actions: false` could inherit it. It is put back after the job's copy.
- `exec` names an origin for the cache v2 it advertises, and masks its runtime token.

Behaviour changes: `no_proxy` no longer exempts `cache.external_server`, and `cache.enabled: false` also stops external registration.

Fixes https://gitea.com/gitea/runner/issues/1208
Fixes https://gitea.com/gitea/runner/issues/1211

Assisted by Claude (Opus 5).

Reviewed-on: https://gitea.com/gitea/runner/pulls/1216
Reviewed-by: bircni <bircni@icloud.com>
Co-authored-by: silverwind <me@silverwind.io>

act/artifactcache/storage.go

@@ -5,8 +5,10 @@
 package artifactcache
 
 import (
+	"bytes"
 	"crypto/sha256"
 	"encoding/hex"
+	"encoding/json/v2"
 	"fmt"
 	"io"
 	"net/http"
@@ -66,6 +68,18 @@ func (s *Storage) WriteBlock(id uint64, blockID string, reader io.Reader) error
 // rename pass is safe because a staged name always carries blockFilePrefix and a target name
 // never does, so no rename can collide with a block not yet moved.
 func (s *Storage) OrderBlocks(id uint64, blockIDs []string) error {
+	wanted, err := json.Marshal(blockIDs) // a block id is client-chosen, so it cannot be a separator
+	if err != nil {
+		return err
+	}
+	list := filepath.Join(s.tempDir(id), blockFilePrefix+"list")
+	if recorded, err := os.ReadFile(list); err == nil {
+		// A retry repeats the list it already sent; another one would reorder what is now staged.
+		if !bytes.Equal(recorded, wanted) {
+			return fmt.Errorf("cache %d was already assembled from a different block list", id)
+		}
+		return nil
+	}
 	for i, blockID := range blockIDs {
 		if err := os.Rename(s.blockName(id, blockID), s.tempName(id, int64(i))); err != nil {
 			if os.IsNotExist(err) {
@@ -74,7 +88,7 @@ func (s *Storage) OrderBlocks(id uint64, blockIDs []string) error {
 			return err
 		}
 	}
-	return nil
+	return os.WriteFile(list, wanted, 0o600)
 }
 
 func (s *Storage) Commit(id uint64, size int64) (int64, error) {