[llvm] [lit] Use .format over format strings (PR #155912)

[Justin Fargnoli via llvm-commits]

justinfargnoli wrote:

@vvereschaka, is it possible for me to test out whether this fixes the issue? 

I attempted to test #154439 before submitting it, but it appeared that my branch needed to be from "llvm-project" (as opposed to my fork of "llvm-project") to be accepted by BuildBot. 

https://github.com/llvm/llvm-project/pull/155912