check out github.ref in ci.yaml (#298)

Using github.sha as the ref in the checkout action prevents git from
seeing the tag when a pushing a tag triggers the workflow, which in turn
causes "brimcap -version" to report a SHA instead of the tag.  Fix by
switching to github.ref.

.github/workflows/ci.yaml

@@ -23,7 +23,7 @@ jobs:
     steps:
       - uses: actions/checkout@v3
         with:
-          ref: ${{ inputs.ref || github.sha }}
+          ref: ${{ inputs.ref || github.ref }}
       - uses: actions/setup-go@v3
         with:
           go-version-file: go.mod