Fix CI changelog validation step (#108)

When checking to see whether the current PR was for a release candidate
or not, we were checking the wrong variable. `github.ref` contains the
branch name if the action was triggered by a branch event, but in this
case it's triggered by a `pull_request` event. For `pull_request`
events, `github.ref` is set to `refs/pulls/[PR number]/merge`, so
nothing was being recognized as a release candidate.

Instead it now checks `github.head_ref`, which is set to the branch
name if it was triggered by `pull_request`. This workflow technically
has two triggers: pushes to the `main` branch, and `pull_request`. But
a push to the main branch is never a release candidate, so it's
sufficient to just check `github.head_ref`.

Also the `yarn setup:postinstall` step was added, as it was missing
from the changelog validation job.

.github/workflows/build-lint-test.yml

@@ -45,11 +45,12 @@ jobs:
           node-version: 12
       - run: yarn --frozen-lockfile
       - run: yarn build
+      - run: yarn setup:postinstall
       - name: Validate RC changelog
-        if: ${{ startsWith(github.ref, 'release/') }}
+        if: ${{ startsWith(github.head_ref, 'release/') }}
         run: yarn changelog validate --rc
       - name: Validate changelog
-        if: ${{ !startsWith(github.ref, 'release/') }}
+        if: ${{ !startsWith(github.head_ref, 'release/') }}
         run: yarn changelog validate
   all-jobs-pass:
     name: All jobs pass