refs #13278 - CI-unixish.yml: do not use xdist with macos-15 in build (mitigates test_color_tty flakiness) (#7774)

Author: firewave

.github/workflows/CI-unixish.yml

@@ -351,9 +351,11 @@ jobs:
         os: [ubuntu-22.04, macos-13, macos-15]
         include:
           - xdist_n: auto
-          # FIXME: test_color_tty fails with xdist
+          # FIXME: test_color_tty fails with xdist - see #13278
           - os: macos-13
             xdist_n: '1'
+          - os: macos-15
+            xdist_n: '1'
       fail-fast: false # Prefer quick result
 
     runs-on: ${{ matrix.os }}