Bug 1531941 [wpt PR 15614] - [wpt] Avoid race condition during module…

… install, a=testonly

Automatic update from web-platform-tests
[wpt] Avoid race condition during module install (#15614)

See web-platform-tests/results-collection#649
--

wpt-commits: cc4f2960306867e50d5beba4e86749aa75961fe8
wpt-pr: 15614

testing/web-platform/tests/tools/wpt/virtualenv.py

@@ -49,7 +49,13 @@ def start(self):
         self.activate()
 
     def install(self, *requirements):
-        call(self.pip_path, "install", *requirements)
+        # `--prefer-binary` guards against race conditions when installation
+        # occurs while packages are in the process of being published.
+        call(self.pip_path, "install", "--prefer-binary", *requirements)
 
     def install_requirements(self, requirements_path):
-        call(self.pip_path, "install", "-r", requirements_path)
+        # `--prefer-binary` guards against race conditions when installation
+        # occurs while packages are in the process of being published.
+        call(
+            self.pip_path, "install", "--prefer-binary", "-r", requirements_path
+        )