CLI: Use BASH if SHELL variable is not set (Windows) (#12847)

lib/python/qmk/commands.py

@@ -238,6 +238,6 @@ def run(command, *args, **kwargs):
         safecmd = map(str, command)
         safecmd = map(shlex.quote, safecmd)
         safecmd = ' '.join(safecmd)
-        command = [os.environ['SHELL'], '-c', safecmd]
+        command = [os.environ.get('SHELL', '/usr/bin/bash'), '-c', safecmd]
 
     return subprocess.run(command, *args, **kwargs)