servo: Merge #17815 - Fix bootstrapping during `mach rustup` without system cargo (from servo:bootstrap); r=nox

https://travis-ci.org/servo/servo-with-rust-nightly/builds/255611949#L989

Source-Repo: https://github.com/servo/servo
Source-Revision: 3854e8f174a834c72bf93c7a76bb33c80021a970

--HG--
extra : subtree_source : https%3A//hg.mozilla.org/projects/converted-servo-linear
extra : subtree_revision : b88e3b9eb099af9b498282afaff79b754460bc78

servo/python/servo/devenv_commands.py

@@ -284,6 +284,9 @@ class MachCommands(CommandBase):
         # Reset self.config["tools"]["rust-root"]
         self._rust_version = None
         self.set_use_stable_rust(False)
+
+        # Reset self.config["tools"]["cargo-root"]
+        self._cargo_build_id = None
         self.set_cargo_root()
 
         self.fetch()