meson: windows: Normalize slashes in prefix
This fixes a build issue on windows, when the prefix is set to a path with
forward slashes. Windows python defaults to a path with a backslash, but mingw
ucrt python defaults to a forward slash. This in turn lead to a wrong PATH set
during tests, causing tests to fail.
Reported-by: Melih Mutlu <m.melihmutlu@gmail.com>
Discussion: http://postgr.es/m/
20220928022724.erzuk5v4ai4b53do@awork3.anarazel.de