fix: Add cargo-install rustup link

Author: epage

text/3502-cargo-script.md

@@ -372,7 +372,7 @@ fn main() {
 ```
 - Like with `cargo install`, `.cargo/config.toml` will be read based on the
   scripts location rather than the current-dir.
-  - And like `cargo install`, the current-dir `rust-toolchain.toml` is respected
+  - And like [`cargo install`](https://github.com/rust-lang/cargo/issues/7312), the current-dir `rust-toolchain.toml` is respected
 - `--release` is not passed in because the primary use case is for exploratory
   programming, so the emphasis will be on build-time performance and debugging,
   rather than runtime performance