Use /cygdrive path prefix for bootstrap-haskell.

[ira-peach]

Fixes #1264.

[hasufell]

I need a way to test this, but I currently have limited capacity to work on CI.

We'd need to extend https://github.com/haskell/ghcup-hs/blob/master/.github/workflows/bootstrap.yaml and then probably use https://github.com/cygwin/cygwin-install-action to set up the test case.

[dfordivam]

I tried to add CI for this in #1267

I did reproduce the issue locally, but unfortunately couldn't reproduce it in CI. So now need to figuring out why CI is behaving different. The CI has MSYS preinstalled so that could be having some effect, not sure.