[test] Fix BugPoint/compile-custom.ll to use detected python exec

Spawn the custom compile command in BugPoint/compile-custom.ll via
%python rather than relying on implicit 'env python' shebang, in order
to fix it on systems that don't have 'python' executable such as NetBSD.

Differential Revision: https://reviews.llvm.org/D55161

llvm-svn: 348095