Fix pnpm cache: use explicit /pnpm-store path and env vars

Mount volume at /pnpm-store and set PNPM_STORE_DIR and
COREPACK_HOME env vars in job containers so pnpm and corepack
both write to the cached volume. Corepack cache avoids
re-downloading pnpm binary each run.

infra/kubernetes/platform/gitea-runner/runner-config.yaml

@@ -11,6 +11,6 @@ data:
       capacity: 1
       timeout: 1h
     container:
-      options: "-v pnpm-store:/root/.local/share/pnpm/store"
+      options: "-v pnpm-store:/pnpm-store -e PNPM_STORE_DIR=/pnpm-store -e COREPACK_HOME=/pnpm-store/.corepack"
       valid_volumes:
         - "pnpm-store"