meson: fix lookup of shell on MINGW64

meson: fix lookup of shell on MINGW64

In 4cba20fbdc6 (meson: prefer shell at "/bin/sh", 2025-04-25) we have
addressed an issue where the shell path embedded into Git was looked up
via PATH, which easily led to unportable shell paths other than the
usual "/bin/sh" location. The fix was to simply add '/bin' to the search
path explicitly, which made us prefer that directory over the PATH-based
lookup.

This fix causes issues on MINGW64 though, which uses Windows-style
paths. "/bin" is not an absolute Windows-style path, but Meson expects
the directories to be absolute. This leads to the following error:

    meson.build:248:15: ERROR: Search directory /bin is not an absolute path.

Fix this by instead searching for both '/bin/sh' and 'sh', which also
causes us to prefer '/bin/sh' over a PATH-based lookup. Meson does
accept that path alright on MINGW64, even though it's not an absolute
Windows-style path, either.

Furthermore, this continues to work alright with cross-files, as well,
in case one wants to explicitly override the shell path:

    $ meson setup build
    ...
      Runtime executable paths
        perl       : /nix/store/gy10hw004rl2xfbfq41vnw0yb1w8rvbl-perl-5.40.0/bin/perl
        python     : /nix/store/sd81bvmch7njdpwx3lkjslixcbj5mivz-python3-3.13.4/bin/python3
        shell      : /bin/sh

    $ cat >cross.ini <<-EOF
    [binaries]
    sh = '/nix/store/94lg0shvsfc845zy8gnflvpqxxiyijbz-bash-interactive-5.2p37/bin/bash'
    EOF

    $ meson setup build --cross-file=cross.ini --wipe
    ...
      Runtime executable paths
        perl       : /nix/store/gy10hw004rl2xfbfq41vnw0yb1w8rvbl-perl-5.40.0/bin/perl
        python     : /nix/store/sd81bvmch7njdpwx3lkjslixcbj5mivz-python3-3.13.4/bin/python3
        shell      : /nix/store/94lg0shvsfc845zy8gnflvpqxxiyijbz-bash-interactive-5.2p37/bin/bash

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>