[CI] Pin older version of importlib-metadata on Linux

(Newer version requires python 3.6, ubuntu 16 has python 3.5)

.github/workflows/build.yml

@@ -77,11 +77,17 @@ jobs:
       warnings: ${{ steps.warnings.outputs.warnings }}
 
     steps:
+      - name: Upgrade pip
+        run: |
+          sudo pip3 install --upgrade pip
+          sudo pip3 install importlib-metadata==2.0.0
       - name: Install Qt
         uses: jurplel/install-qt-action@v2
         with:
           version: ${{ env.qt_version }}
           aqtversion: ==0.9.2
+        env:
+          PIP_IGNORE_REQUIRES_PYTHON: yes
       - name: Download InstallBuilder
         run: |
           wget -O ~/installbuilder.run ${{ env.bitrock_url }}/installbuilder-${{ env.bitrock_version }}-linux-x64-installer.run