chore: refactor act/artifactcache Handler to an interface (#934)

- the Handler struct becomes handler (lowercase)
- the Handler interface is defined to be the existing methods
- isClosed() is added and used only in tests
- setgcAt() is added and used only in tests

---

This is to allow mocking the Handler interface for testing.

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  - [PR](https://code.forgejo.org/forgejo/runner/pulls/934): <!--number 934 --><!--line 0 --><!--description Y2hvcmU6IHJlZmFjdG9yIGFjdC9hcnRpZmFjdGNhY2hlIEhhbmRsZXIgdG8gYW4gaW50ZXJmYWNl-->chore: refactor act/artifactcache Handler to an interface<!--description-->
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Reviewed-on: https://code.forgejo.org/forgejo/runner/pulls/934
Reviewed-by: Mathieu Fenniak <mfenniak@noreply.code.forgejo.org>
Co-authored-by: Earl Warren <contact@earl-warren.org>
Co-committed-by: Earl Warren <contact@earl-warren.org>

internal/app/cmd/exec.go

@@ -54,7 +54,7 @@ type executeArgs struct {
 	debug                 bool
 	dryrun                bool
 	image                 string
-	cacheHandler          *artifactcache.Handler
+	cacheHandler          artifactcache.Handler
 	network               string
 	enableIPv6            bool
 	githubInstance        string