chore: use blobless partial clone in prebuild checkout

Address review feedback on the shallow-clone approach:

- fetch-depth: 1/2 breaks .github/actions/version which needs
  git describe --tags --long main on the full commit graph.
- changed-files@v45 detected the shallow clone and spent ~30s
  deepening it itself, negating the checkout speedup.

Switch to fetch-depth: 0 + filter: blob:none (blobless partial
clone) which keeps the full commit graph and tags but skips the
expensive blob download. This gives git describe what it needs
while cutting checkout time.

The conditional checkout skip for release builds (workflow_call
with version supplied) remains — no prebuild step needs the repo
on that path.

Author: decentraland-bot

.github/workflows/build-unitycloud.yml

@@ -259,7 +259,8 @@ jobs:
         if: github.event_name == 'pull_request' || (github.event.inputs.version == '' && inputs.version == '')
         uses: actions/checkout@v6
         with:
-          fetch-depth: ${{ github.event_name == 'pull_request' && 1 || 2 }}
+          fetch-depth: 0
+          filter: blob:none
 
       - name: Detect changes under Explorer/**
         if: github.event_name == 'pull_request'