Disable macos on GitHub Actions

PyCall requires that Python is build with `--enable-shared` option.
GitHub Action's macOS image has Python but it isn't build with
`--enable-shared`, so PyCall cannot be tested on that environment.

Author: mrkn

.github/workflows/ci.yml

@@ -13,7 +13,7 @@ jobs:
       matrix:
         os:
         - ubuntu-18.04
-        - macos-latest
+        #- macos-latest
         ruby_version:
         - 2.6.x
         - 2.5.x