`get_test_bin` may be able to be undeprecated in an upcoming Cargo release

[epage]

rust-lang/cargo#16421 made the compile-time environment variables available at runtime. This has been approved but isn't technically stable until release as things can still be reverted. In assert_cmd, I added a fallback to support this in case this does get stabilized but I'm holding off on un-deprecating until it is stabilized, see https://docs.rs/assert_cmd/latest/src/assert_cmd/cargo.rs.html#220-251

Found from this crater run: rust-lang/rust#149852

[MichaelMcDonnell]

Thank you Ed for the in-depth explanation and the heads up!