enhance: add runner.tool_cache_mode and default it to none (#1171)

The current shared tools cache is not concurrency-safe, e.g. multiple jobs can write and corrupt it, for example `setup-go` with explicit go version under concurrency reliably corrupts the tool cache and fails all jobs.

This adds a new `runner.tool_cache_mode` (and `--tool-cache-mode` exec option) option which defaults to unshared tools cache:

- `none` mounts nothing, so a job uses what its image ships there and discards what it installs
- `shared` keeps the single volume every job reuses, and warns when `runner.capacity` is above 1

Under `none` effective tool cache can only come from the image or host, which is the same as it is on GitHub Actions which ships many preinstalled tools in its fat VM images.

Co-authored-by: bircni <bircni@icloud.com>
Reviewed-on: https://gitea.com/gitea/runner/pulls/1171
Reviewed-by: bircni <bircni@icloud.com>
Co-authored-by: silverwind <me@silverwind.io>

internal/app/run/runner.go

@@ -159,8 +159,8 @@ func (r *Runner) OnIdle(ctx context.Context) {
 	}
 	// Host mode: reclaim per-job scratch dirs left behind when HostEnvironment
 	// cleanup timed out (e.g. a delete stalled by an AV/EDR filter driver). They
-	// sit under the host workdir parent alongside the shared tool_cache, which
-	// the name match leaves untouched. No-op when no host-mode job ever ran.
+	// sit under the host workdir parent next to a shared tool_cache, which the name
+	// match leaves untouched. No-op when no host-mode job ever ran.
 	if hostRoot := filepath.FromSlash(r.cfg.Host.WorkdirParent); hostRoot != "" {
 		r.cleanupStaleDirs(ctx, hostRoot, isHostScratchDir)
 	}
@@ -219,7 +219,7 @@ func isTaskIDDir(name string) bool {
 // isHostScratchDir reports whether name is a per-job host-mode scratch dir:
 // hex.EncodeToString of 8 random bytes, i.e. exactly 16 lowercase hex chars
 // (see startHostEnvironment in act/runner/run_context.go). The narrow match
-// leaves the sibling shared "tool_cache" dir and any operator data untouched.
+// leaves a sibling shared "tool_cache" dir and any operator data untouched.
 func isHostScratchDir(name string) bool {
 	if len(name) != 16 {
 		return false
@@ -522,6 +522,7 @@ func (r *Runner) run(ctx context.Context, task *runnerv1.Task, reporter *report.
 		JobCompletedHook:                  r.cfg.Runner.Hooks.JobCompleted,
 		Vars:                              task.Vars,
 		ValidVolumes:                      r.cfg.Container.ValidVolumes,
+		SharedToolCache:                   r.cfg.Runner.ToolCacheMode == config.ToolCacheModeShared,
 		InsecureSkipTLS:                   r.cfg.Runner.Insecure,
 		RunnerName:                        r.name,
 	}