meson: windows: Fix tmp_install + prefix computation with meson 1.0.1

In d0366bfb3b21 I said:
    Instead of trying to do this in meson.build, call out to the implementation
    meson install uses. This isn't pretty, but it's more reliable than what we had
    before.

Unfortunately it was too ugly - to fix a bug, meson 1.0.1 changed the way the
meson internal runpython helper works, resulting in the previous sys.argv[]
indices not working anymore.  Just open-code it - it's just a few characters
longer.

Committing this quickly to allow ci/cfbot to work with meson 1.0.1 on windows.

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