Allow setting CFG_DISABLE_UNSTABLE_FEATURES to 0

Two locations check whether this build-time environment variable is
defined. Allowing it to be explicitly disabled with a "0" value is
useful, especially for integrating with external build systems.

library/test/src/cli.rs

@@ -309,7 +309,8 @@ fn parse_opts_impl(matches: getopts::Matches) -> OptRes {
 // FIXME: Copied from librustc_ast until linkage errors are resolved. Issue #47566
 fn is_nightly() -> bool {
     // Whether this is a feature-staged build, i.e., on the beta or stable channel
-    let disable_unstable_features = option_env!("CFG_DISABLE_UNSTABLE_FEATURES").is_some();
+    let disable_unstable_features =
+        option_env!("CFG_DISABLE_UNSTABLE_FEATURES").map(|s| s != "0").unwrap_or(false);
     // Whether we should enable unstable features for bootstrapping
     let bootstrap = env::var("RUSTC_BOOTSTRAP").is_ok();