ci: rename jobs key to macos

Fixes: 56ceca8 (ci: add a macOS GitHub Actions workflow)

Author: lzaoral

.github/workflows/macos.yml

@@ -8,7 +8,7 @@ on:
     branches: [main]
 
 jobs:
-  ubuntu:
+  macos:
     name: macOS ${{ matrix.version }} (${{ matrix.compiler }})
     strategy:
       fail-fast: false