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.

substrate/Cargo.lock

@@ -2007,6 +2007,18 @@ dependencies = [
  "log",
 ]
 
+[[package]]
+name = "filetime"
+version = "0.2.16"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "c0408e2626025178a6a7f7ffc05a25bc47103229f19c113755de7bf63816290c"
+dependencies = [
+ "cfg-if 1.0.0",
+ "libc",
+ "redox_syscall",
+ "winapi",
+]
+
 [[package]]
 name = "finality-grandpa"
 version = "0.15.0"
@@ -10648,6 +10660,7 @@ dependencies = [
  "ansi_term",
  "build-helper",
  "cargo_metadata",
+ "filetime",
  "sp-maybe-compressed-blob",
  "strum",
  "tempfile",