fix: don't suggest replacing `env!("CARGO_BIN_NAME")` with itself
Some environment variables (e.g. `CARGO_BIN_NAME`) are only available for some targets ([Cargo Targets](https://doc.rust-lang.org/cargo/reference/cargo-targets.html), [§ Environment variables Cargo sets for crates](https://doc.rust-lang.org/cargo/reference/environment-variables.html#environment-variables-cargo-sets-for-crates)), and the current diagnostic when copy-pasting code from a binary to a library is kinda confusing:
```rs
const _: &str = env!("CARGO_BIN_NAME");
```
Before:
```
error: environment variable `CARGO_BIN_NAME` not defined at compile time
--> lib.rs:1:17
|
1 | const _: &str = env!("CARGO_BIN_NAME");
| ^^^^^^^^^^^^^^^^^^^^^^
|
= help: there is a similar Cargo environment variable: `CARGO_BIN_NAME`
```
After:
```
error: environment variable `CARGO_BIN_NAME` not defined at compile time
--> lib.rs:1:17
|
1 | const _: &str = env!("CARGO_BIN_NAME");
| ^^^^^^^^^^^^^^^^^^^^^^
|
= help: `CARGO_BIN_NAME` may not be available for the current Cargo target
= help: Cargo sets build script variables at run time. Use `std::env::var("CARGO_BIN_NAME")` instead
```
@rustbot label +T-compiler +A-diagnostics +D-confusing
Jonathan Brouwer
86063080a5