Rebuild test-runtime if substrate binary is updated (#362)

* Re-run test-runtime build script if the substrate binary changes

* cargo fmt

* Can => Cannot

test-runtime/Cargo.toml

@@ -12,3 +12,4 @@ codec = { package = "parity-scale-codec", version = "2", default-features = fals
 subxt = { path = ".." }
 sp-core = { package = "sp-core", git = "https://github.com/paritytech/substrate/", branch = "master" }
 async-std = { version = "1.9.0", features = ["attributes", "tokio1"] }
+which = "4.2.2"

test-runtime/build.rs

@@ -113,6 +113,14 @@ async fn run() {
     fs::write(&runtime_path, runtime_api_contents)
         .expect("Couldn't write runtime rust output");
 
+    let substrate_path =
+        which::which(substrate_bin).expect("Cannot resolve path to substrate binary");
+
+    // Re-build if the substrate binary we're pointed to changes (mtime):
+    println!(
+        "cargo:rerun-if-changed={}",
+        substrate_path.to_string_lossy()
+    );
     // Re-build if we point to a different substrate binary:
     println!("cargo:rerun-if-env-changed={}", SUBSTRATE_BIN_ENV_VAR);
     // Re-build if this file changes: