As of #1079 the VSCode cargo-watch functionality has been broken on
Linux systems.
The cause seems to be that linux takes the added quotes inside process
arguments literally, so it attempts to make cargo-watch run the command
`cargo "check --message-format json"` with the entire quoted part being
treated as a single long subcommand, which cargo doesn't know how to
handle.
Removing the extra quotes solves the issue.
4d78c85de2