ci: Upgrade github actions macOS runner

  The `macos-10.15` runner is depecrated since 5/31/22 and will be
  unsupported by 12/1/22 (see
  actions/runner-images#5583).

  We did not upgrade before because the `macos-11` runner does not
  provide VirtualBox which we need to create an Ext filesystem for
  `cozy-stack` (see
  actions/runner-images#4010 for more details on
  previous attempts to add it to `macos-11`)
  and it was not yet available in the `macos-12` runner.

  It's now available in `macos-12` (see
  actions/runner-images#5594) so we can upgrade
  to `macos-12`.

.github/workflows/macos.yaml

@@ -36,7 +36,7 @@ jobs:
 
   unit:
     needs: cancel
-    runs-on: macos-10.15
+    runs-on: macos-12
     name: Unit tests
     strategy:
       matrix:
@@ -90,7 +90,7 @@ jobs:
 
   integration:
     needs: cancel
-    runs-on: macos-10.15
+    runs-on: macos-12
     name: Integration tests
     strategy:
       matrix:
@@ -144,7 +144,7 @@ jobs:
 
   scenarios:
     needs: cancel
-    runs-on: macos-10.15
+    runs-on: macos-12
     name: Scenarios
     strategy:
       matrix:
@@ -206,7 +206,7 @@ jobs:
 
   build:
     needs: cancel
-    runs-on: macos-10.15
+    runs-on: macos-12
     name: Build packages
     steps:
       - uses: actions/checkout@v3