debug: fix short-form custom command name on --invoke isn't used

[ktock]

This fixes the bug that --invoke ignores command name specified as the short-form argument. Sorry, this seems introduced in #1970 that uses InvokeConfig.NoCmd in an incorrect way in the short-form flag 🙏 .

master branch:

Command name (ps) on the short-form flag is ignored.

$ BUILDX_EXPERIMENTAL=1 /tmp/out/buildx build --invoke ps /tmp/ctx3/
...
Launching interactive container. Press Ctrl-a-c to switch to monitor console
Interactive container was restarted with process "a41yfa5q4d20h1ezin01ndcun". Press Ctrl-a-c to switch to the new container
root@buildkitsandbox:/# 

This PR:

Command name (ps) is enabled.

$ BUILDX_EXPERIMENTAL=1 /tmp/out/buildx build --invoke ps /tmp/ctx3/
...
Launching interactive container. Press Ctrl-a-c to switch to monitor console
Interactive container was restarted with process "su15exc88brmlsj7ae7c77nzv". Press Ctrl-a-c to switch to the new container
    PID TTY          TIME CMD
      1 pts/0    00:00:00 ps