fix: build rstest explicitly after install

Upstream moved `pnpm run build` out of rstest's root `prepare` script
(web-infra-dev/rstest@569c2ce), so `pnpm i` alone no longer builds
`workspace/rstest`. Add an explicit `node --run build` mirroring
build-rsbuild so @rstest/core ships its dist + type declarations and
downstream consumer tsc can resolve it.

Author: fi3ework

.github/actions/build-rstest/action.yaml

@@ -40,3 +40,4 @@ runs:
       run: |
         cd ${{ inputs.path }}
         pnpm i
+        node --run build