Fix pkg installer var in deep-dive code snippet

The workflow was reverted to source APPLE_DEVELOPER_INSTALLER_ID from
vars.APPLE_DEVELOPER_INSTALLER_ID, but the docs snippet still showed
vars.MAC_APP_SIGNING_IDENTITY.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: niik

docs/release-process-deep-dive.md

@@ -191,7 +191,7 @@ There is no signing of linux artifacts in this job. See the [release job](#relea
         if: inputs.environment == 'production'
         env:
           TAG_NAME: ${{ inputs.tag_name }}
-          APPLE_DEVELOPER_INSTALLER_ID: ${{ vars.MAC_APP_SIGNING_IDENTITY }}
+          APPLE_DEVELOPER_INSTALLER_ID: ${{ vars.APPLE_DEVELOPER_INSTALLER_ID }}
         run: |
           shopt -s failglob
           script/pkgmacos "$TAG_NAME"