fix: don't suggest replacing env!("CARGO_BIN_NAME") with itself

[ShE3py]

Some environment variables (e.g. CARGO_BIN_NAME) are only available for some crate types, and the current diagnostic when copy-pasting code from a binary to a library is kinda confusing:

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

[rustbot]

r? @davidtwco

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