aya-build: enable -Zbuild-std=core when RUSTC_BOOTSTRAP is set

aya-build/src/lib.rs

@@ -146,7 +146,21 @@ pub fn build_ebpf<'a>(
             &target,
         ]);
 
-        if channel == Channel::Nightly {
+        // RUSTC_BOOTSTRAP controls whether unstable features (-Z flags) are available:
+        //   "1"           – enable for all crates
+        //   "<crate>"     – enable only for the named crate
+        //   "-1"          – force stable behavior even on nightly (explicit opt-out)
+        //   (unset/other) – follow the actual compiler channel
+        // See: https://doc.rust-lang.org/beta/unstable-book/compiler-environment-variables/RUSTC_BOOTSTRAP.html
+        let rustc_bootstrap = env::var("RUSTC_BOOTSTRAP").unwrap_or_default();
+        let use_build_std = if rustc_bootstrap == "-1" {
+            false
+        } else if rustc_bootstrap == "1" || rustc_bootstrap == name {
+            true
+        } else {
+            channel == Channel::Nightly
+        };
+        if use_build_std {
             cmd.args(["-Z", "build-std=core"]);
         }