mempool: minor refactorings

- Use `sourceRoot`
- Allow `generate.sh` to directly trigger the build of `nodeModules`.
  Needed by mempool 3.2.1, where `mempool-backend` has multiple fixed output
  derivation dependencies, which can only be updated if they can be
  built directly.

pkgs/mempool/generate.sh

@@ -50,7 +50,7 @@ updateNodeModulesHash() {
     component=$1
     echo
     echo "Fetching node modules for mempool-$component"
-    ../../helper/update-fixed-output-derivation.sh ./default.nix mempool-"$component" "cd $component"
+    ../../helper/update-fixed-output-derivation.sh ./default.nix mempool-"$component".nodeModules "sourceRoot.*$component"
 }
 
 updateFrontendAssets() {