https://rust-analyzer.github.io/book/configuration.html#check.overrideCommand claims I can provide a string:

But this didn't work as expected, so I had to check source code and my suspicion was correct, it expects a Vec:

https://github.com/rust-lang/rust-analyzer/blob/8611b714597c89b092f3d4874f14acd3f72f44fd/crates/rust-analyzer/src/config.rs#L844C9-L844C99