Add gha-creds-*.json ignore (#1644)

## Type of change

Internal bugfix.

### What should this PR do?

Prevent short-term token leakage.

### Why are we making this change?

It's required to securely use the google-github-actions/auth
documentation: https://github.com/google-github-actions/auth - though no
technical control is in place to ensure the existence of this entry.

See google-github-actions/auth#298 for more
context.

### What are the acceptance criteria? 

That it doesn't break tests.

### How should this PR be tested?

Signed-off-by: ltagliaferri <lisa.tagliaferri@gmail.com>
Co-authored-by: ltagliaferri <lisa.tagliaferri@gmail.com>

.gitignore

@@ -18,4 +18,8 @@ terraform.tfstate.backup
 terraform.tfstate.*.backup
 .terraform.tfstate.lock.*
 .terraform.lock.hcl
+
+# required by google-github-actions
+gha-creds-*.json
+
 /tools/rumble/vulnerability.db