Appveyor doesn't like clean-cargo-cache changes

[jdm]

set CARGO_HOME=C:\Users\appveyor\.cargo
mach clean-nightlies --keep 2 --force
Current Rust version for Servo: nightly-2018-01-21
Current Rust version for geckolib: 1.22.1
Nothing to remove.
mach clean-cargo-cache --keep 2 --force
Error running mach:
    ['clean-cargo-cache', '--keep', '2', '--force']
The error occurred in the implementation of the invoked mach command.
This should never occur and is likely a bug in the implementation of that
command. Consider filing a bug for this issue.
If filing a bug, please include the full output of mach, including this error
message.
The details of the failure are as follows:
WindowsError: [Error 3] The system cannot find the path specified: u'C:\\Users\\appveyor\\.cargo\\git\\db\\*.*'
  File "C:\projects\servo\python\servo\bootstrap_commands.py", line 238, in clean_cargo_cache
    git_db_list = filter(lambda f: not f.startswith('.'), os.listdir(git_db_dir))
Command exited with code 1

[highfive]

cc @aneeshusa

[pyfisch]

Probably related: #19823

[tigercosmos]

Previous work, but I don't know why fail now.

[jdm]

I'm pretty sure this error means that we don't actually cache the .cargo directory on appveyor.