Only list the first build directory
Only list the first build directory

At some point, Xcode started putting this setting in multiple places. This
caused the subsequent commands to fail because we were trying to use `find`
with a wacky string that included newlines.

To fix this, we can use the `-m` flag for grep to only display the first
result.

This is probably fragile, but progress is progress and this entire thing is a
horrifying hack anyway.

gfontenot/vim-xcodebuild,gfontenot/vim-xcode