ci: avoid using the deprecated set-env construct

The `set-env` construct was deprecated as of the announcement in
https://github.blog/changelog/2020-10-01-github-actions-deprecating-set-env-and-add-path-commands/

Let's use the recommended alternative instead.

Signed-off-by: Johannes Schindelin <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: dscho

.github/workflows/check-whitespace.yml

@@ -14,7 +14,7 @@ jobs:
     steps:
     - name: Set commit count
       shell: bash
-      run: echo "::set-env name=COMMIT_DEPTH::$((1+$COMMITS))"
+      run: echo "COMMIT_DEPTH=$((1+$COMMITS))" >>$GITHUB_ENV
       env:
         COMMITS: ${{ github.event.pull_request.commits }}