Use SSH deploy key for submodule checkout in CI

- Add ssh-key parameter to checkout action in build.yml and release.yml
- Uses DEPLOY_KEY secret for SSH authentication to internal repos
- Fixes 403 permission errors when cloning mods/silo submodule

.gitea/workflows/build.yml

@@ -33,6 +33,7 @@ jobs:
         with:
           submodules: recursive
           fetch-depth: 0
+          ssh-key: ${{ secrets.DEPLOY_KEY }}
 
       - name: Install pixi if needed
         shell: bash

.gitea/workflows/release.yml

@@ -42,6 +42,7 @@ jobs:
           submodules: recursive
           fetch-depth: 0
           fetch-tags: true
+          ssh-key: ${{ secrets.DEPLOY_KEY }}
 
       - name: Install pixi if needed
         shell: bash