fix(ci): trial macos-26 public preview.

See if the test failures subside on the latest public preview of the
MacOS runners.

Author: tm91236

.github/workflows/unittests.yml

@@ -21,7 +21,7 @@ jobs:
         os:
           - ubuntu-latest
           - windows-latest
-          - macos-latest
+          - macos-26
     runs-on: ${{ matrix.os }}
     env:
       # Disable implicitly syncing before running - we run an explicit sync first.