fetch-yarn-deps: reference files to be copied directly

This would cause any drv to be rebuilt when any part of the default.nix changes.
What we actually care about is the two JS files though, so simply reference them
directly.

Co-authored-by: Infinidoge <infinidoge@inx.moe>

pkgs/build-support/node/fetch-yarn-deps/default.nix

@@ -25,7 +25,8 @@ in {
 
       tar --strip-components=1 -xf ${yarnpkg-lockfile-tar} package/index.js
       mv index.js $out/libexec/yarnpkg-lockfile.js
-      cp ${./.}/common.js ${./.}/index.js $out/libexec/
+      cp ${./common.js} $out/libexec/common.js
+      cp ${./index.js} $out/libexec/index.js
 
       patchShebangs $out/libexec
       makeWrapper $out/libexec/index.js $out/bin/prefetch-yarn-deps \
@@ -53,7 +54,8 @@ in {
 
       tar --strip-components=1 -xf ${yarnpkg-lockfile-tar} package/index.js
       mv index.js $out/libexec/yarnpkg-lockfile.js
-      cp ${./.}/common.js ${./.}/fixup.js $out/libexec/
+      cp ${./common.js} $out/libexec/common.js
+      cp ${./fixup.js} $out/libexec/fixup.js
 
       patchShebangs $out/libexec
       makeWrapper $out/libexec/fixup.js $out/bin/fixup-yarn-lock