On Windows based systems (e.g. msys), running PKGBUILD linting is very
slow due to time taken spawning bash subshells.  Additionally, some packages
have extreme amounts of (usually procedurally generated) splitting, which
also causes linting to be extremely slow.  Provide an environment variable
to disable PKGBUILD linting.

Signed-off-by: Allan McRae <allan@archlinux.org>