ci: Switch macOS back to v10.15

  The `macos-latest` Github Actions virtual environment does not support
  VirtualBox anymore (see
  actions/runner-images#4060 and
  actions/runner-images#4010 for more
  details).

  Since we do need a virtual machine running Linux for the cozy-stack to
  have access to a unix filesystem and mimic our production VFS
  behavior, we have to switch back to using the `macos-10.15` virtual
  environment.

  Apparently Github is looking for a solution to this issue so once they
  do find one, we'll be able to switch again to the `macos-latest`
  environment.

.github/workflows/macos.yaml

@@ -38,7 +38,7 @@ jobs:
 
   unit:
     needs: cancel
-    runs-on: macos-latest
+    runs-on: macos-10.15
     name: Unit tests
     strategy:
       matrix:
@@ -91,7 +91,7 @@ jobs:
 
   integration:
     needs: cancel
-    runs-on: macos-latest
+    runs-on: macos-10.15
     name: Integration tests
     strategy:
       matrix:
@@ -144,7 +144,7 @@ jobs:
 
   scenarios:
     needs: cancel
-    runs-on: macos-latest
+    runs-on: macos-10.15
     name: Scenarios
     strategy:
       matrix:
@@ -205,7 +205,7 @@ jobs:
 
   build:
     needs: cancel
-    runs-on: macos-latest
+    runs-on: macos-10.15
     name: Build packages
     steps:
       - uses: actions/checkout@v2