wasm-builder: Fix constant re-running of `build.rs` scripts. (#11624)

Recently we added the wasm binaries to the `rerun-if-changed` list. The problem with that is that
they have a later mtime than the `invoked.timestamp` file and this file's mtime is used to determine
if the `build.rs` script needs to be re-run. The solution to this is that we copy the mtime of this
`invoked.timestamp` file and add it to the wasm binaries. Then cargo/rustc doesn't constantly wants
to rerun the `build.rs` script.