chore: Use continuous-integration environment for private submodule a…

…ccess (#714)

Co-authored-by: Robin Salkeld <salkeldr@amazon.com>

.github/workflows/osx.yml

@@ -7,6 +7,7 @@ on: ["pull_request", "push"]
 jobs:
   OSX:
     runs-on: ${{ matrix.os }}
+    environment: continuous-integration
 
     strategy:
       matrix:
@@ -20,6 +21,14 @@ jobs:
         uses: actions/checkout@v2
         with:
           submodules: recursive
+        if: ${{ github.repository == 'aws/aws-encryption-sdk-c' }}
+
+      - name: Checkout PR with CI bot token
+        uses: actions/checkout@v2
+        with:
+          submodules: recursive
+          token: ${{ secrets.CI_BOT_TOKEN }}
+        if: ${{ github.repository == 'aws/private-aws-encryption-sdk-c-staging' }}
 
       - name: Checkout AWS C++ SDK
         uses: actions/checkout@v2