Right now, if executing `xcrun` fails, then the error message prints the
second argument to the `xcrun.py` script, which is the first argument to
the tool that `xcrun` is executing, making the whole error message quite
confusing.
Consider the following error:
```
python ../../third_party/squirrel.mac/build/xcrun.py dtrace -h -s /private/tmp/20210531211008-def376dc/src/third_party/squirrel.mac/vendor/ReactiveObjC/ReactiveObjC/RACSignalProvider.d -o /private/tmp/20210531211008-def376dc/src/out/release/gen/third_party/squirrel.mac/dtrace/RACSignalProvider.h
xcrun script '-h' failed with code '71':
xcrun: error: can't exec '/tmp/20210531211008-def376dc/dtrace' (errno=Permission denied)
```
The command that `xcrun` is executing is `dtrace`, but the error just
mentions the `-h` flag.
Notes: none
Signed-off-by: Juan Cruz Viotti <jv@jviotti.com>
d6d9a6a039