Fix cache: combine export + pnpm install in single step

.gitea/workflows/deploy-preview.yaml

@@ -10,13 +10,10 @@ jobs:
     steps:
       - uses: actions/checkout@v4
 
-      - name: Setup pnpm
+      - name: Setup and install
         run: |
           export COREPACK_HOME=/pnpm-store/.corepack
           corepack enable && corepack prepare pnpm@latest --activate
-
-      - name: Install dependencies
-        run: |
           export PNPM_STORE_DIR=/pnpm-store
           pnpm install --frozen-lockfile