Doc builds are busted since the switch to rustup

[jdm]

Every doc build shows:

Error running mach:

    ['doc']

The error occurred in code that was called by the mach command. This is either
a bug in the called code itself or in the way that mach is calling it.

You should 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:

OSError: [Errno 2] No such file or directory

  File "/home/servo/buildbot/slave/doc/build/python/servo/post_build_commands.py", line 214, in doc
    rustc_path = check_output(["rustup" + BIN_SUFFIX, "which", "rustc"], env=env)
  File "/home/servo/buildbot/slave/doc/build/python/servo/command_base.py", line 153, in check_output
    return subprocess.check_output(*args, shell=sys.platform == 'win32', **kwargs)
  File "/usr/lib/python2.7/subprocess.py", line 566, in check_output
    process = Popen(stdout=PIPE, *popenargs, **kwargs)
  File "/usr/lib/python2.7/subprocess.py", line 710, in __init__
    errread, errwrite)
  File "/usr/lib/python2.7/subprocess.py", line 1327, in _execute_child
    raise child_exception
program finished with exit code 1
elapsedTime=10.037575

[highfive]

cc @aneeshusa

[SimonSapin]

My guess is that we need something like servo/saltfs#762 for the doc build job.