Skip Ruby head on CI (ubuntu and macos)

These aren't passing because `mutex_m` is no longer included by default
in Ruby's standard library. Need to upgrade `minitest` to get it to
work, but can't do that yet because the new version doesn't support Ruby
2.5.

Related: #178

.github/workflows/ci.yml

@@ -8,6 +8,10 @@ jobs:
         os: [ubuntu-latest, windows-latest, macos-latest]
         ruby: [2.5, 2.6, 2.7, 3.0, 3.1, 3.2, 3.3, head, jruby, jruby-head, truffleruby, truffleruby-head]
         exclude:
+          - os: ubuntu-latest
+            ruby: head
+          - os: macos-latest
+            ruby: head
           - os: windows-latest
             ruby: truffleruby
           - os: windows-latest