[polly][llvm-lit] Enabled lit internal shell for polly test suite (#105704)

This patch sets lit's internal shell to be the default shell when
running polly tests.

This is one of the milestones to resolving the meta-issue:
https://github.com/llvm/llvm-project/issues/102704.

GitOrigin-RevId: 6b86f51b98197420b3d9bb44a1d9ae3d3b118df8