Explicitly install runtime library packages in install-php action

Previously the shared libraries would not be available when PHP was cached,
because the step that installs the dev packages containing the headers (which
would implicitly install the library package) was skipped.

.github/actions/install-php/action.yml

@@ -16,6 +16,15 @@ runs:
       shell: bash
       run: sudo apt-get update
 
+    - name: Install runtime libraries
+      shell: bash
+      run: |
+        sudo apt-get install -y \
+          libmcrypt4 \
+          libmemcached11 \
+          librabbitmq4 \
+          librdkafka1
+
     - name: Determine PHP version
       id: determine-php-version
       shell: bash