chromium: fix rpath-overwriting in WideVine plugin

pkgs/applications/networking/browsers/chromium/plugins.nix

···
       ! find -iname '*.so' -exec ldd {} + | grep 'not found'
     '';
 
-    patchPhase = ''
-      for sofile in libwidevinecdm.so libwidevinecdmadapter.so; do
-        chmod +x "$sofile"
-        patchelf --set-rpath "${mkrpath [ stdenv.cc.cc glib nspr nss ]}" "$sofile"
-      done
+    PATCH_RPATH = mkrpath [ stdenv.cc.cc glib nspr nss ];
 
-      patchelf --set-rpath "$out/lib:${mkrpath [ stdenv.cc.cc ]}" \
-        libwidevinecdmadapter.so
+    patchPhase = ''
+      chmod +x libwidevinecdm.so libwidevinecdmadapter.so
+      patchelf --set-rpath "$PATCH_RPATH" libwidevinecdm.so
+      patchelf --set-rpath "$out/lib:$PATCH_RPATH" libwidevinecdmadapter.so
     '';
 
     installPhase = let