CI: cache the Appose cache

So that every single build doesn't need to download and unpack
micromamba, then download and unpack all conda environment packages.

.github/workflows/build.yml

@@ -34,6 +34,15 @@ jobs:
         with:
           python-version: '3.10'
 
+    - name: Cache Appose environments
+      id: cache-appose
+      uses: actions/cache@v4
+      with:
+        path: ~/.local/share/appose
+        key: ${{ runner.os }}-build-appose-${{ hashFiles('*') }}
+        restore-keys: |
+          ${{ runner.os }}-build-appose-
+
       - name: Set up CI environment
         run: .github/setup.sh