GH Actions: automatically retry postfix install on failure

The postfix installation step fails regularly, resulting in failed CI builds which have to be restarted, while the failure is not due to anything in the PR.

This commit introduces a new action runner for the postfix install, which will automatically retry the install up to 3 times.

If it works as I expect it to, this should eliminate failed CI builds due to postfix installs erroring out.

Ref: https://github.com/marketplace/actions/retry-step

.github/workflows/tests.yml

@@ -131,10 +131,17 @@ jobs:
         with:
           composer-options: --ignore-platform-reqs
 
+      # Install postfix and automatically retry if the install failed, which happens reguarly.
+      # @link https://github.com/marketplace/actions/retry-step
       - name: Install postfix
-        run: |
-          sudo apt-get install -y -qq postfix qmail
-          sudo service postfix stop
+        uses: nick-invision/retry@v2
+        with:
+          timeout_minutes: 2
+          max_attempts: 3
+          retry_wait_seconds: 5
+          command: |
+            sudo apt-get install -y -qq postfix qmail
+            sudo service postfix stop
 
       - name: Set up sendmail
         run: |
@@ -145,7 +152,7 @@ jobs:
           sudo mkdir -p /var/qmail/bin
           sudo cp test/fakesendmail.sh /var/qmail/bin/sendmail
           sudo cp test/fakesendmail.sh /usr/sbin/sendmail
-      
+
       - name: Run tests, no code coverage
         if: ${{ matrix.coverage == false }}
         run: ./vendor/bin/phpunit --no-coverage