Switches from `cd js && make ...` to `make -C js ...` so buildPhase
leaves pwd at the source root. installPhase's `js/Imaging.{js,wasm}`
paths are now correct against an explicit pwd rather than relying on
buildPhase's mid-phase cd side-effect carrying over.
No functional change — just robustness against future phase reorders /
`dontBuild` overrides, per argus's 🟡 on the v2 review of #697.
0231ac84d3