[ci] cache downloads

Author: JohannesBuchner

.github/workflows/testing.yml

@@ -1,5 +1,7 @@
 name: Build
 on: [push, pull_request]
+env:
+  CACHE_NUMBER: 0  # increase to reset cache manually
 jobs:
   run-tests:
     name: Run tests
@@ -17,6 +19,14 @@ jobs:
         with:
           auto-update-conda: true
           python-version: ${{ matrix.python-version }}
+      - name: Set cache date
+        run: echo "DATE=$(date +'%Y%m%d')" >> $GITHUB_ENV
+      - name: Conda download cache
+        id: myconda-download-cache
+        uses: actions/cache@v3
+        with:
+          path: /usr/share/miniconda/pkgs/
+          key: ${{ matrix.python-version }}-conda-${{ hashFiles('requirements*.txt') }}-${{ env.DATE }}-${{ env.CACHE_NUMBER }}
       - name: Install imagemagick
         run: |
           sudo apt-get install -y imagemagick libstdc++6