GHA CI: Move global environment variables out

.github/workflows/ci.yaml

@@ -8,8 +8,6 @@ on: [pull_request, push]
 
 env:
   VCPKG_COMMIT: 8dddc6c899ce6fdbeab38b525a31e7f23cb2d5bb
-  VCPKG_DEST_MACOS: /Users/runner/qbt_tools/vcpkg
-  VCPKG_DEST_WIN: C:\qbt_tools\vcpkg
 
 jobs:
   ubuntu:
@@ -90,6 +88,9 @@ jobs:
     name: Windows
     runs-on: windows-2019
 
+    env:
+      VCPKG_DEST_WIN: C:\qbt_tools\vcpkg
+
     defaults:
       run:
         shell: pwsh
@@ -183,6 +184,9 @@ jobs:
         qbt_gui: ["GUI=ON", "GUI=OFF"]
       fail-fast: false
 
+    env:
+      VCPKG_DEST_MACOS: /Users/runner/qbt_tools/vcpkg
+
     defaults:
       run:
         shell: pwsh