Assert release & nightly builds are signed, notarised & accessible before deployment (#559)

.github/workflows/build_macos.yaml

@@ -88,6 +88,14 @@ jobs:
                   CSC_KEY_PASSWORD: ${{ secrets.APPLE_CSC_KEY_PASSWORD }}
                   CSC_LINK: ${{ secrets.APPLE_CSC_LINK }}
 
+            - name: Check app was signed & notarised successfully
+              if: inputs.sign != ''
+              run: |
+                  hdiutil attach dist/*.dmg
+                  codesign -dv --verbose=4 /Volumes/Element*/*.app
+                  spctl -a -vvv -t install /Volumes/Element*/*.app
+                  hdiutil detach /Volumes/Element*
+
             - name: "[Unsigned] Build App"
               if: inputs.sign == ''
               run: |