CI: macOS: re-enable fuse2 testing, work around #6099

Downgrade to macOS 10.15 as macOS 11.x breaks fuse2 testing.

On macOS 10.15, osxfuse and also macfuse works with borg's fuse2 tests.

Upgrading to macOS 11.x based github CI breaks the tests, see #6099,
even when using the more recent macfuse to provide fuse2 capability.

.github/workflows/ci.yml

@@ -60,9 +60,9 @@ jobs:
             - os: ubuntu-20.04
               python-version: '3.10'
               toxenv: py310-fuse3
-            - os: macos-latest
+            - os: macos-10.15  # macos-latest is macos 11.6.2 and hanging at test_fuse, #6099
               python-version: '3.8'
-              toxenv: py38-none
+              toxenv: py38-fuse2
 
     env:
       # Configure pkg-config to use OpenSSL from Homebrew
@@ -107,7 +107,7 @@ jobs:
         brew install zstd || brew upgrade zstd
         brew install lz4 || brew upgrade lz4
         brew install openssl@1.1 || brew upgrade openssl@1.1
-        #brew install homebrew/cask/osxfuse || brew upgrade homebrew/cask/osxfuse  # Required for Python llfuse module
+        brew install --cask macfuse || brew upgrade --cask macfuse  # Required for Python llfuse module
 
     - name: Install Python requirements
       run: |