test: fix the build test

temp_dir() returns the root temp directory and not a new one.

Author: hansl

dfx/Cargo.lock

@@ -32,3 +32,4 @@ wabt = "0.9.2"
 [dev-dependencies]
 env_logger = "0.6"
 mockito = "0.20.0"
+tempfile = "3.1.0"

dfx/Cargo.toml

@@ -180,7 +180,8 @@ mod tests {
         let env = TestEnv {};
         let wat = r#"(module )"#;
 
-        let temp_path = temp_dir();
+        let temp_dir = tempfile::tempdir().unwrap();
+        let temp_path = temp_dir.into_path();
         let input_path = temp_path.join("input.wat");
         let output_path = temp_path.join("output.wasm");