chore(ci): check out PR code for acceptance testing (#381)

When I updated the acceptance test workflow to allow them to run against
external PRs (by using the `pull_request_target` trigger), I forgot to
update the workflow to actually check out the PR. The
`pull_request_target` workflow runs in the context of the base branch,
so if it isn't explicitly told to check out something else, it will
check out and run the tests from `main` instead of from the PR.

.github/workflows/acctest.yml

@@ -41,7 +41,9 @@ jobs:
     steps:
 
     - name: Check out code into the Go module directory
-      uses: actions/checkout@v3
+      uses: actions/checkout@v4
+      with:
+          ref: ${{ github.event.pull_request.head.sha || github.ref }}
 
     - name: Set up Go
       uses: actions/setup-go@v4