Add ac claim for old docker/build-push-action@v3 / current buildx gha cache (#29584)

Also resolves a warning for current releases

```
| ##[group]GitHub Actions runtime token ACs
| ##[warning]Cannot parse GitHub Actions Runtime Token ACs: "undefined" is not valid JSON
| ##[endgroup]
====>
| ##[group]GitHub Actions runtime token ACs
| ##[endgroup]
```
\* this is an error in v3

References in the docker org:
-
831ca179d3/src/main.ts (L24)
-
7d8b4dc669/src/github.ts (L61)

No known official action of GitHub makes use of this claim.

Current releases throw an error when configure to use actions cache
```
| ERROR: failed to solve: failed to configure gha cache exporter: invalid token without access controls
| ##[error]buildx failed with: ERROR: failed to solve: failed to configure gha cache exporter: invalid token without access controls
```

services/actions/auth_test.go

@@ -7,6 +7,7 @@ import (
 	"net/http"
 	"testing"
 
+	"code.gitea.io/gitea/modules/json"
 	"code.gitea.io/gitea/modules/setting"
 
 	"github.com/golang-jwt/jwt/v5"
@@ -29,6 +30,14 @@ func TestCreateAuthorizationToken(t *testing.T) {
 	taskIDClaim, ok := claims["TaskID"]
 	assert.True(t, ok, "Has TaskID claim in jwt token")
 	assert.Equal(t, float64(taskID), taskIDClaim, "Supplied taskid must match stored one")
+	acClaim, ok := claims["ac"]
+	assert.True(t, ok, "Has ac claim in jwt token")
+	ac, ok := acClaim.(string)
+	assert.True(t, ok, "ac claim is a string for buildx gha cache")
+	scopes := []actionsCacheScope{}
+	err = json.Unmarshal([]byte(ac), &scopes)
+	assert.NoError(t, err, "ac claim is a json list for buildx gha cache")
+	assert.GreaterOrEqual(t, len(scopes), 1, "Expected at least one action cache scope for buildx gha cache")
 }
 
 func TestParseAuthorizationToken(t *testing.T) {