Fix enabling cfg(gc_zeal) during fuzzing (#13123)

This commit fixes a minor mistake from #13053 where `cfg!(fuzzing)`
isn't set for build scripts because `RUSTFLAGS`, where `--cfg fuzzing`
is specified, is only passed to target artifacts, not build scripts.
This means that the lookup must happen via Cargo's set env vars (e.g.
`CARGO_CFG_FUZZING`), not `cfg!(...)`.

Author: alexcrichton

crates/cranelift/build.rs

@@ -1,5 +1,5 @@
 fn main() {
-    if cfg!(fuzzing) {
+    if std::env::var("CARGO_CFG_FUZZING").is_ok() {
         println!("cargo:rustc-cfg=gc_zeal");
     }
 }

crates/wasmtime/build.rs

@@ -34,7 +34,7 @@ fn main() {
     custom_cfg("has_virtual_memory", has_virtual_memory);
     custom_cfg("has_custom_sync", has_custom_sync);
     custom_cfg("has_host_compiler_backend", has_host_compiler_backend);
-    custom_cfg("gc_zeal", cfg!(fuzzing));
+    custom_cfg("gc_zeal", cfg("fuzzing"));
 
     // If this OS isn't supported and no debug-builtins or if Cranelift doesn't support
     // the host or there's no need to build these helpers.