feat(mobile): add yarn cache to prepare job in release pipeline

Add cache: 'yarn' to actions/setup-node@v4 in the prepare job of
build.yml. This mirrors the pattern already used in setup-node-modules.yml
and reduces yarn install --immutable from ~2m 30s to ~50s on cache hit.

Part of MCWP-574

Author: Qbandev

.github/workflows/build.yml

@@ -125,6 +125,7 @@ jobs:
         uses: actions/setup-node@v4
         with:
           node-version-file: '.nvmrc'
+          cache: 'yarn'
       - run: yarn install --immutable
       - run: node scripts/validate-build-config.js