fix: issue with rerun from compilation buffer

[sunng87]

This patch updates how we treat subcommand and full command in cargo-mode--start. This issue happens when using envrc mode if we trigger run last command from the compilation buffer, it gets different PATH from original mode. Then it results in prepending another cargo command to previous full command.

The patch changes the behaviour when running last command, it doesn't attempt to check if it's full command.