[llvm] [libcxx] [ci] Make the CI find the right version of Clang-cl (PR #161736)

[Martin Storsjö via llvm-commits]

mstorsjo wrote:

> Aside from this, it seems like at least the `clang-cl-dll` test config also fails `std/input.output/filesystems/fs.op.funcs/fs.op.proximate/proximate.pass.cpp`. No idea why that happens; we'll see if that's a fluke if we get the other issues sorted out.

That did indeed seem to be a fluke; now all the Windows builds succeeded again - so I'll merge this, to unblock https://github.com/llvm/llvm-project/pull/134330.

https://github.com/llvm/llvm-project/pull/161736