fix(rust-analyzer): don't run cargo if Cargo.toml is not found

[kitterion]

After commit d5d7412 I started getting the message

[lspconfig] cmd failed with code 101: table: 0x0104f90fe0
error: could not find `Cargo.toml` in `<path>` or any parent directory
Press ENTER or type command to continue

every time I open a rust file in a project that doesn't use cargo. For some reason cargo is launched even if Cargo.toml is not found which seems wrong, so I moved all the cargo logic inside if cargo_crate_dir ~= nil.
rust-analyzer supports non-cargo projects via rust-project.json that nvim-lspconfig also supports, and it's what I use.

[glepnir]

i don't think this is correct. This command is still available when cargo_crate_dir does not exist. Try running this command cargo metadata --no-deps --format-version=1

[kitterion]

I get the exact same error because there's no Cargo.toml

$ cargo metadata --no-deps --format-version=1
error: could not find `Cargo.toml` in `<path>` or any parent directory

[glepnir]

okay.. command must run with cargo.toml. thanks.