win32: mach and build command fixes

- Add SERVO_USE_NIGHTLY_RUST env var to use the latest rust/cargo nightly snapshot
- Fix up looking for cargo binary (in cargo/bin/cargo, not bin/cargo)
- Fix up win32 executable checking (use .exe suffix)
- fix up win32 PATH handling (subprocess must use shell=True for PATH change to be honored)

python/servo/build_commands.py

@@ -11,7 +11,6 @@ from __future__ import print_function, unicode_literals
 
 import os
 import os.path as path
-import subprocess
 import sys
 import shutil
 
@@ -23,7 +22,7 @@ from mach.decorators import (
     Command,
 )
 
-from servo.command_base import CommandBase, cd
+from servo.command_base import CommandBase, cd, call
 
 
 def is_headless_build():
@@ -123,14 +122,6 @@ def notify(title, text):
             print("[Warning] Could not generate notification! %s" % extra, file=sys.stderr)
 
 
-def call(*args, **kwargs):
-    """Wrap `subprocess.call`, printing the command if verbose=True."""
-    verbose = kwargs.pop('verbose', False)
-    if verbose:
-        print(' '.join(args[0]))
-    return subprocess.call(*args, **kwargs)
-
-
 @CommandProvider
 class MachCommands(CommandBase):
     @Command('build',