Only test macOS on stable

Most of the advantages from testing various Rust versions already come
from running those tests on Linux and Windows. There's very little
gain from also running these tests on macOS, while macOS jobs are the
slowest to schedule.

Signed-off-by: Joe Richey <joerichey@google.com>

.github/workflows/tests.yml

@@ -39,8 +39,12 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        os: [ubuntu-latest, macos-latest, windows-latest]
+        os: [ubuntu-latest, windows-latest]
         toolchain: [nightly, beta, stable, 1.34]
+        # Only Test macOS on stable to reduce macOS CI jobs
+        include:
+          - os: macos-latest
+            toolchain: stable
     steps:
       - uses: actions/checkout@v2
       - uses: actions-rs/toolchain@v1