Respect RUSTC_BOOTSTRAP=-1 in rustc version detection

build.rs

@@ -563,7 +563,11 @@ mod version {
             }
             let minor = digits.next()?.parse::<u32>().ok()?;
             let _patch = digits.next().unwrap_or("0").parse::<u32>().ok()?;
-            let nightly = channel == "nightly" || channel == "dev";
+            let nightly = match env::var_os("RUSTC_BOOTSTRAP") {
+                // When -1 is passed rustc works like stable, e.g., cfg(target_feature = "unstable_target_feature") will never be set. /~https://github.com/rust-lang/rust/pull/132993
+                Some(v) if v == "-1" => false,
+                _ => channel == "nightly" || channel == "dev",
+            };
 
             // Note that rustc 1.49-1.50 (and 1.13 or older) don't print LLVM version.
             let llvm_major = (|| {