prefix gets lost on config regen for cross Windows build

[Matheus Tavares Bernardino]

[resending as it looks like there was some delivery issue with the first
msg]

Hi,

It seems that we lose the install prefix option when regenerating config
for a Windows cross-build. Looks like this behavior appeared with c36dd41ba2
(configure: move target-specific defaults to an external machine file,
2023-10-16), but I couldn't find the specific root cause yet.

To reproduce the issue, first run:

    ../configure --prefix=/tmp/install --without-default-features \
        --cross-prefix=x86_64-w64-mingw32- --target-list=aarch64-softmmu

Which correctly prints:

    Directories
        [...]
        Install prefix: /tmp/install

But then if we run `touch ../meson.build && make`, we get:

    Directories
        [...]
        Install prefix: /qemu

Removing the `prefix = '/qemu'` line from configs/meson/windows.txt does
fix the issue, but I don't understand why the CLI option is not taking
precedence over it.... Or even if this could be a meson bug itself. I'd
appreciate any help.

Thanks,
Matheus