set `Custom-Time` metadata on GCS cache object when retrieving

### Description of the feature request:

When bazel retrieves an object from GCS cache bucket, set the [Custom-Time](https://cloud.google.com/storage/docs/metadata#custom-time) metadata on the object to the datetime of the retrieval.

### Which category does this issue belong to?

Remote Execution

### What underlying problem are you trying to solve with this feature?

Would like to use the GCS lifecycle condition [daysSinceCustomTime](https://cloud.google.com/storage/docs/lifecycle#dayssincecustomtime) to delete objects that are not recently retrieved to keep the bucket size down and reduce storage costs.

Note: the [age](https://cloud.google.com/storage/docs/lifecycle#age) lifecycle condition does not exactly do this because it is triggered in days from creation, not retrieval.

### Which operating system are you running Bazel on?

linux

### What is the output of `bazel info release`?

_No response_

### If `bazel info release` returns `development version` or `(@non-git)`, tell us how you built Bazel.

_No response_

### What's the output of `git remote get-url origin; git rev-parse HEAD` ?

_No response_

### Have you found anything relevant by searching the web?

_No response_

### Any other information, logs, or outputs that you want to share?

Thoughts on implementation: the `daysSinceCustomTime` is very coarse and so it may not be important to set `Custom-Time` very precisely. Perhaps rounding retrieval datetime to the nearest hour and if the `Custom-Time` on a retrieved object is already set to that hour, then no need to set it again.