ci: Upload puppeteer artifacts on failure.

Storing the puppeteer artifacts is useful for debugging failures in
CI.

Confusingly, `if: ${{ something }}` does not work out to be true like
`if: ${{ always() && something }}` does; the former has a silent
`success()` built into it[1]:

> If your if expression does not contain any of the status functions
> it will automatically result with success().

[1] https://docs.github.com/en/actions/reference/context-and-expression-syntax-for-github-actions#job-status-check-functions

.github/workflows/zulip-ci.yml

@@ -143,7 +143,8 @@ jobs:
           pip install codecov && codecov || echo "Error in uploading coverage reports to codecov.io."
 
       - name: Store Puppeteer artifacts
-        if: ${{ matrix.include_frontend_tests }}
+        # Upload these on failure, as well
+        if: ${{ always() && matrix.include_frontend_tests }}
         uses: actions/upload-artifact@v2
         with:
           name: puppeteer