Set CARGO_BIN_NAME environment variable also for binary examples

[t-rapp]

What does this PR try to resolve?

The existing CARGO_BIN_NAME environment variable is useful when building command-line programs. It lets the command know its binary name, e.g. for printing out simple usage instructions. This PR extends the CARGO_BIN_NAME environment variable to be set when building binary example targets, additional to "normal" binary targets.

Fixes #11689.

How should we test and review this PR?

Create a new binary project with cargo new hello-env. Add a new file "examples/foo-bar.rs" with the following lines:

fn main() {
    let program = env!("CARGO_BIN_NAME");
    println!("{program}");
}

Building and running the example with cargo run --example foo-bar should print a line with the string "foo-bar".

Note that this PR extends the existing testcase for the CARGO_BIN_NAME environment variable with an example target.

[rustbot]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @ehuss (or someone else) soon.

Please see the contribution instructions for more information.

[epage]

@bors r+

[bors]

📌 Commit 0fda7ac has been approved by epage

It is now in the queue for this repository.

[bors]

⌛ Testing commit 0fda7ac with merge f0a4ee0...

[bors]

☀️ Test successful - checks-actions
Approved by: epage
Pushing f0a4ee0 to master...

[t-rapp]

Thanks for the review!