build: Let Meson figure out Python installation
Commit 4a4d9eb6 initially switched Meson to find the python program using find_program('python3'). Sadly that caused a regression, since in some cases with MSVC it would fallback to 'meson.exe runpython', which is undesired.
However, that particular code was reverted back to an also undesired lookup method, find_installation('python3'). This way of finding python also breaks on Windows + MSVC, in particular when setting it up as follows:
winget install python
winget install meson
This fails building GLib with:
python3 not found
Fix that by not passing any argument to find_installation(), which lets Meson figure it all out by itself.