environment: Use os.path.basename() when checking mesonlib.meson_command

Without this, when one of Meson's parent directories contains
"python", unexpected option "-u" will be passed to Meson.

mesonbuild/environment.py

@@ -375,7 +375,7 @@ class Environment:
 
     def get_build_command(self, unbuffered=False):
         cmd = mesonlib.meson_command[:]
-        if unbuffered and 'python' in cmd[0]:
+        if unbuffered and 'python' in os.path.basename(cmd[0]):
             cmd.insert(1, '-u')
         return cmd