Fix secrets detection build step to improve branch detection

[aduth]

🛠 Summary of changes

Fixes failing builds caused by git failures in the script.

[git] fatal: ambiguous argument 'origin/main..HEAD': unknown revision or path not in the working tree.

Borrows liberally from changelog script logic for git branching:

identity-idp/.gitlab-ci.yml

Lines 140 to 152 in 00e4701

	- |
	if [ "$CI_MERGE_REQUEST_TARGET_BRANCH_NAME" == "main" ]
	then
	git fetch origin --quiet
	./scripts/changelog_check.rb -b origin/"${CI_MERGE_REQUEST_TARGET_BRANCH_NAME}" -s origin/"${CI_MERGE_REQUEST_SOURCE_BRANCH_NAME}"
	elif [ "$CI_EXTERNAL_PULL_REQUEST_TARGET_BRANCH_NAME" == "main" ]
	then
	git fetch origin --quiet
	./scripts/changelog_check.rb -b origin/"${CI_EXTERNAL_PULL_REQUEST_TARGET_BRANCH_NAME}" -s origin/"${CI_EXTERNAL_PULL_REQUEST_SOURCE_BRANCH_NAME}"
	else
	echo "Skipping because this is not a PR or is not targeting main"
	exit 0
	fi

Discussion: https://gsa-tts.slack.com/archives/C0NGESUN5/p1696511447586139?thread_ts=1696289942.572699&cid=C0NGESUN5

Previous PR: #9296

📜 Testing Plan

Build should pass.

[mitchellhenke]

There's a few line changes like this, are they needed?

[aduth]

There's a few line changes like this, are they needed?

Not strictly necessary, they're from my Prettier auto-formatter.

We apply Prettier already to a good chunk of the YAML files in this repository and I'd feel semi-strongly that it should be applied / enforced here as well, but absent the tooling for that, I think the occasional manual "sync-up" is okay?

[soniaconnolly]

LGTM, thanks for fixing!