Fix trybuild version

More recent versions of trybuild do not appear to respect the RUSTFLAGS
we set just before performing the build. This means they fail to find
libautocxx-demo.a and linking fails.

Author: adetaylor

Cargo.lock

@@ -20,7 +20,7 @@ categories = ["development-tools::ffi", "api-bindings"]
 
 [dependencies]
 proc-macro2 = "1.0.11"
-trybuild = "1.0.53"
+trybuild = "=1.0.81"
 test-log = "0.2.2"
 env_logger = "0.9.0"
 rust_info = "0.3.1"