generic-worker: Indexed Artifacts shouldn't be cached

[petemoore]

Describe the bug
Indexed Artifacts were added in release v51.0.0, initially broken, and then fixed in v51.1.0. You can mount an artifact in a generic worker payload by specifying an index namespace and artifact name, together with a filename or directory name to mount the artifact at. Unlike other content that can be mounted (artifact content, url content, raw content, base64 content) the content should not be assumed to be up-to-date if already cached on disk. Since the index can be updated over time, we should keep a reference to the taskID in the cache, and if the taskID has changed, the content should be re-downloaded. Note, technically the content could change if a task was resolved successfully, and then was rerun and for some reason also was successful but produced a different artifact, but that is a very unusual edge case that we probably don't want to support.

To Reproduce

Steps to reproduce the behavior:

1. Start generic-worker
2. Mount an artifact from the index
3. Update the index entry to point to a different taskID with the same artifact name, but with different contents
4. Mount the same artifact from the index
5. See that the mount in step 4 is from the content from the original index entry, not the updated one

Expected behavior
The artifact mounted should be from the task that was in the index under the given namespace at the time the task ran

Taskcluster version
51.1.0

Additional context
But discovered while working on #6271. Will also be fixed in this PR. Test TestIndexedArtifact tests for this issue.

[petemoore]

Duplicate of #6476 (whoops)