Add "cargo:rerun-if-changed=build.rs" to some build.rs files.

Add "cargo:rerun-if-changed=build.rs" in some build.rs files to tell
cargo that it doesn't need to scan the whole package. See the
[Cargo docs] for more info.

[Cargo docs]: https://doc.rust-lang.org/cargo/reference/build-scripts.html#rerun-if-changed

crates/misc/run-examples/build.rs

@@ -3,4 +3,5 @@ fn main() {
         "cargo:rustc-env=TARGET={}",
         std::env::var("TARGET").unwrap()
     );
+    println!("cargo:rerun-if-changed=build.rs");
 }

crates/wasi-common/build.rs

@@ -7,4 +7,6 @@ fn main() {
     println!("cargo:wasi={}", wasi.display());
     // and available to our own crate as WASI_ROOT:
     println!("cargo:rustc-env=WASI_ROOT={}", wasi.display());
+    // and this build.rs script doesn't depend on any files.
+    println!("cargo:rerun-if-changed=build.rs");
 }