Use explicit slash-dot for copy paths

[jefferbrecht]

Description

Follow-on to #1761. filepath.Join(directory, ".") can trim the dot away which messes up the gcloud config copy operation; this PR adds the dot explicitly without using filepath.Join.

Related issue

b/352054649

How has this been tested?

Tested and succeeded locally.

Checklist:

• Unit tests
  • Unit tests do not apply.
  • Unit tests have been added/modified and passed for this PR.
• Integration tests
  • Integration tests do not apply.
  • Integration tests have been added/modified and passed for this PR.
• Documentation
  • This PR introduces no user visible changes.
  • This PR introduces user visible changes and the corresponding documentation change has been made.
• Minor version bump
  • This PR introduces no new features.
  • This PR introduces new features, and there is a separate PR to bump the minor version since the last release already.
  • This PR bumps the version.

[igorpeshansky]

LGTM

[jefferbrecht]

Failures look unrelated; merging.