fix(ci): go_tooling CI job git checkout tip of PR (#173)

## Why this should be merged

The release-branch CI job tests branch properties of the `git` DAG but
GitHub's `actions/checkout` by default creates a speculative merge
commit against which PR CI is run. Running against this hypothetical
situation makes the tests fail.

## How this works

[`actions/checkout` uses PR
tip](https://github.com/actions/checkout?tab=readme-ov-file#checkout-pull-request-head-commit-instead-of-merge-commit)

## How this was tested

Future run against a release branch.

---------

Signed-off-by: Quentin McGaw <[email protected]>

Author: qdm12

.github/workflows/go.yml

@@ -49,6 +49,7 @@ jobs:
       - uses: actions/checkout@v4
         with:
           fetch-depth: 0 # everything
+          ref: ${{ github.event.pull_request.head.sha }}
       - name: Set up Go
         uses: actions/setup-go@v5
         with: