nix/hive-matrix: drop implicit cd in fluffychat-web-imaging buildPhase (argus #697 v2 🟡)

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.

nix/modules/hive-matrix.nix

@@ -81,16 +81,18 @@ let
       export HOME=$TMPDIR
       export EM_CACHE=$TMPDIR/.emscriptencache
       mkdir -p $EM_CACHE
-      cd js
-      make Imaging.js Imaging.wasm
+      # `make -C js` keeps the build phase pwd at the source root so
+      # installPhase doesn't have to know about the cd (argus 🟡 on
+      # PR #697 v2 — robust against future reorders / `dontBuild`).
+      make -C js Imaging.js Imaging.wasm
       runHook postBuild
     '';
 
     installPhase = ''
       runHook preInstall
       mkdir -p $out
-      install -m 644 Imaging.js  $out/Imaging.js
-      install -m 644 Imaging.wasm $out/Imaging.wasm
+      install -m 644 js/Imaging.js  $out/Imaging.js
+      install -m 644 js/Imaging.wasm $out/Imaging.wasm
       runHook postInstall
     '';