meson: prefer 'python3' to 'python' when finding python3

Although in some cases python 3.x will come without "3" as suffix,
"python3" should still be preferred because sometimes "python" is python
2.x instead.

Signed-off-by: Icenowy Zheng <uwu@icenowy.me>
Reviewed-by: Eric Engestrom <None>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/33025>

meson.build

@@ -917,7 +917,7 @@ if get_option('allow-kcmp') \
 endif
 
 # On Windows, a venv has no versioned aliased to 'python'.
-prog_python = find_program('python', 'python3', version : '>= 3.8')
+prog_python = find_program('python3', 'python', version : '>= 3.8')
 
 has_mako = run_command(
   prog_python, '-c',