Make requirements compile disable pip's require-virtualenv flag always

Setting explicitly to 0 is required when it's done through the config file.

Author: adamchainz

requirements/compile.py

@@ -9,7 +9,7 @@
 if __name__ == "__main__":
     os.chdir(Path(__file__).parent)
     os.environ["CUSTOM_COMPILE_COMMAND"] = "requirements/compile.py"
-    os.environ.pop("PIP_REQUIRE_VIRTUALENV", None)
+    os.environ["PIP_REQUIRE_VIRTUALENV"] = "0"
     common_args = [
         "-m",
         "piptools",