Build/Test Tools: Remove shell: bash from code coverage workflow.

bash is the default shell for all non-Windows GitHub Action runners, so this is not necessary.

Follow up to [51670].
See #53363.

git-svn-id: https://develop.svn.wordpress.org/trunk@51671 602fd350-edb4-49c9-b593-d223f7449a82

.github/workflows/test-coverage.yml

@@ -90,7 +90,6 @@ jobs:
       - name: "Get last Monday's date"
         id: get-date
         run: echo "::set-output name=date::$(/bin/date -u --date='last Mon' "+%F")"
-        shell: bash
 
       - name: Get Composer cache directory
         id: composer-cache