meson: drop explicit python interpreter

meson: drop explicit python interpreter

meson wraps python scripts already on win32, so we end up with these
failing commands:

[1/359] "C:/msys64/ucrt64/bin/meson" "--internal" "exe" "--capture" "src/util/virkeycodetable_atset1.h" "--" "sh" "C:/msys64/home/marca/src/libvirt/scripts/meson-python.sh" "C:/msys64/ucrt64/bin/python3.EXE" "python" "C:/msys64/home/marca/src/libvirt/src/keycodemapdb/tools/keymap-gen" "code-table" "--lang" "stdc" "--varname" "virKeyCodeTable_atset1" "C:/msys64/home/marca/src/libvirt/src/keycodemapdb/data/keymaps.csv" "atset1"
FAILED: src/util/virkeycodetable_atset1.h
"C:/msys64/ucrt64/bin/meson" "--internal" "exe" "--capture" "src/util/virkeycodetable_atset1.h" "--" "sh" "C:/msys64/home/marca/src/libvirt/scripts/meson-python.sh" "C:/msys64/ucrt64/bin/python3.EXE" "python" "C:/msys64/home/marca/src/libvirt/src/keycodemapdb/tools/keymap-gen" "code-table" "--lang" "stdc" "--varname" "virKeyCodeTable_atset1" "C:/msys64/home/marca/src/libvirt/src/keycodemapdb/data/keymaps.csv" "atset1"

If LC_ALL, LANG and LC_CTYPE need to be set, it would probably be better
to use a meson environment() instead.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>