Fix host cleanup, volume allowlist, cache upload, and action host edge cases (#970)

## Summary
- prevent host-mode execution from deleting caller-owned workdirs
- harden `valid_volumes` checks against `..` and symlink escapes
- return immediately after artifact cache upload write failures
- default implicit remote action clone hosts to `GitHubInstance`/`github.com`

Authored with assistance from OpenAI Codex GPT-5.

---------

Co-authored-by: silverwind <me@silverwind.io>
Reviewed-on: https://gitea.com/gitea/runner/pulls/970
Reviewed-by: silverwind <2021+silverwind@noreply.gitea.com>

act/runner/run_context.go

@@ -220,12 +220,12 @@ func (rc *RunContext) startHostEnvironment() common.Executor {
 		}
 		toolCache := filepath.Join(cacheDir, "tool_cache")
 		rc.JobContainer = &container.HostEnvironment{
-			Path:        path,
-			TmpDir:      runnerTmp,
-			ToolCache:   toolCache,
-			Workdir:     rc.Config.Workdir,
-			BindWorkdir: rc.Config.BindWorkdir,
-			ActPath:     actPath,
+			Path:         path,
+			TmpDir:       runnerTmp,
+			ToolCache:    toolCache,
+			Workdir:      rc.Config.Workdir,
+			CleanWorkdir: rc.Config.CleanWorkdir,
+			ActPath:      actPath,
 			CleanUp: func() {
 				os.RemoveAll(miscpath)
 			},