fix: attach task token when cloning actions from self-hosted instance on a different host (#1056)

When `DEFAULT_ACTIONS_URL=self`, action clone URLs (`uses: owner/repo@ref`) are
built from the Gitea **AppURL** (`gitea_default_actions_url`), but
`shouldCloneURLUseToken` compared the clone URL host only against the runner's
**registered address** (`GitHubInstance`).

When the runner registers with a different hostname than AppURL — same instance,
different DNS (e.g. `gitea.local` vs `gitea.my-nas.lan`, internal vs external) —
the strict `u1.Host == u2.Host` check returns false, so the task token is **not**
attached and the action clone goes out anonymously. Against an instance with
`REQUIRE_SIGNIN_VIEW=true` this fails with:

```
Unable to clone https://gitea.example/owner/action refs/heads/v1: authentication required
```

The current workaround is to make the runner's registered host exactly match
`AppURL`. This PR removes the need for that.

Refs: https://github.com/go-gitea/gitea/issues/27933

## Change

- `shouldCloneURLUseToken` now trusts the clone URL when its host matches **either**
  the registered instance (`GitHubInstance`) **or** the self-hosted default-actions
  instance (`DefaultActionInstance`). Embedded basic auth is still rejected, and the
  empty-host cases are unchanged.
- A new `Config.DefaultActionInstanceIsSelfHosted` flag gates the second candidate.
  It is set in the daemon layer (`run/runner.go`, `exec.go`), where `github.com` and
  a configured `GithubMirror` are distinguishable, so the token is **never** attached
  for off-instance hosts.Reviewed-on: https://gitea.com/gitea/runner/pulls/1056
Reviewed-by: Zettat123 <39446+zettat123@noreply.gitea.com>
Co-authored-by: bircni <bircni@icloud.com>

internal/app/cmd/exec.go

@@ -443,10 +443,11 @@ func runExec(ctx context.Context, execArgs *executeArgs) func(cmd *cobra.Command
 			NoSkipCheckout:        execArgs.noSkipCheckout,
 			// PresetGitHubContext:   preset,
 			// EventJSON:             string(eventJSON),
-			ContainerNamePrefix:   "GITEA-ACTIONS-TASK-" + eventName,
-			ContainerMaxLifetime:  maxLifetime,
-			ContainerNetworkMode:  container.NetworkMode(execArgs.network),
-			DefaultActionInstance: execArgs.defaultActionsURL,
+			ContainerNamePrefix:               "GITEA-ACTIONS-TASK-" + eventName,
+			ContainerMaxLifetime:              maxLifetime,
+			ContainerNetworkMode:              container.NetworkMode(execArgs.network),
+			DefaultActionInstance:             execArgs.defaultActionsURL,
+			DefaultActionInstanceIsSelfHosted: execArgs.defaultActionsURL != "" && execArgs.defaultActionsURL != "https://github.com",
 			PlatformPicker: func(_ []string) string {
 				return execArgs.image
 			},