miqt-docker: Git might be Cygwin/msys git, should do path conversion

Currently miqt-docker determines the repo's root path using a combination of go.mod, git root, and so on. If the git binary in $PATH is Cygwin/MSYS git, it might supply a Cygwin/MSYS path that should be converted back to a GOOS-native path before it can be used.