Override always 'CARGO_HOME' env variable.

This use always `cargo-home-dir` in `/.servobuild`
even if you set `CARGO_HOME` in your shell.

If you use [racer][racer] with [rustup (multirust.rs)][rustup],
you may set `CARGO_HOME` env variable.
Then the previous code would be a problem which does not use
`cargo-home-dir` in `/.servobuild`.

[racer]: https://github.com/phildawes/racer
[rustup]: https://github.com/rust-lang-nursery/rustup.rs

python/servo/command_base.py

@@ -293,8 +293,7 @@ def build_env(self, gonk=False, hosts_file_path=None):
         if extra_path:
             env["PATH"] = "%s%s%s" % (os.pathsep.join(extra_path), os.pathsep, env["PATH"])
 
-        if "CARGO_HOME" not in env:
-            env["CARGO_HOME"] = self.config["tools"]["cargo-home-dir"]
+        env["CARGO_HOME"] = self.config["tools"]["cargo-home-dir"]
 
         if "CARGO_TARGET_DIR" not in env:
             env["CARGO_TARGET_DIR"] = path.join(self.context.topdir, "target")