Don’t override PATH when calling xcodebuild

The builds on my continuous integration server are failing because they rely on the `PATH` to be set appropriately, and `xctool` overrides it.

I tried removing this line, and all the tests pass:

    ./xctool.sh -workspace xctool.xcworkspace -scheme xctool test
    
    [lots of output]
    
    ** TEST SUCCEEDED: 191 passed, 0 failed, 0 errored, 191 total ** (43912 ms)

If this is not desired, I have another change that would add `/usr/bin`, `/bin`, `/usr/sbin`, and `/sbin` to the `PATH` instead of overriding the `PATH` entirely.