fix: perform initial gcloud check and reuse token

[kurtisvg]

Change Description

Reverts behavior to verify a gcloud token is available before using it as a TokenSource, and caches the token when possible.

Checklist

• Make sure to open an issue as a
  bug/issue
  before writing your code! That way we can discuss the change, evaluate
  designs, and agree on the general idea.
• Ensure the tests and linter pass
• Appropriate documentation is updated (if necessary)

Relevant issues:

• Fixes 1.20.1 image crashes with: errors parsing config: exec: "gcloud": executable file not found in $PATH #655