allow exec command after --workspace-folder flag

[n-arakawa]

It seems that the 'exec' command is only allowed as the first parameter.

 devcontainer  --workspace-folder . exec  ls
OCI runtime exec failed: exec failed: unable to start container process: exec: "exec": executable file not found in $PATH: unknown

Like up etc., it expects options followed by exec to be allowed.

With this change, one can create an alias as alias devc="devcontainer --workspace-folder PATH".