Be less unix-centric in error messages

Co-authored-by: Eric Huss <eric@huss.org>

Author: jyn514

src/cargo/core/compiler/custom_build.rs

@@ -598,7 +598,7 @@ impl BuildOutput {
                             // Abort with an error.
                             anyhow::bail!("Cannot set `RUSTC_BOOTSTRAP={}` from {}.\n\
                                 note: Crates cannot set `RUSTC_BOOTSTRAP` themselves, as doing so would subvert the stability guarantees of Rust for your project.\n\
-                                help: If you're sure you want to do this in your project, use `RUSTC_BOOTSTRAP={} cargo build` instead.\n\
+                                help: If you're sure you want to do this in your project, set the environment variable `RUSTC_BOOTSTRAP={}` before running cargo instead.\n\
                                 help: See https://doc.rust-lang.org/cargo/reference/build-scripts.html#rustc-env for details.",
                                 val,
                                 whence,

tests/testsuite/build_script_env.rs

@@ -122,7 +122,7 @@ fn rustc_bootstrap() {
         .build();
     p.cargo("build")
         .with_stderr_contains("error: Cannot set `RUSTC_BOOTSTRAP=1` [..]")
-        .with_stderr_contains("help: [..] use `RUSTC_BOOTSTRAP=foo cargo build` [..]")
+        .with_stderr_contains("help: [..] set the environment variable `RUSTC_BOOTSTRAP=foo` [..]")
         .with_status(101)
         .run();
     p.cargo("build")