Configurable download url

[picoHz]

Hello.
I want to use a mirror server for downloading static libraries in build.rs because downloading files from GitHub is extremely slow in my home.

Could you add an option to change download url like this?

--- a/build.rs
+++ b/build.rs
@@ -168,7 +168,8 @@ fn download_ninja_gn_binaries() {
 }

 fn static_lib_url() -> (String, String) {
-  let base = "https://github.com/denoland/rusty_v8/releases/download";
+  let default_base = "https://github.com/denoland/rusty_v8/releases/download";
+  let base = env::var("RUSTY_V8_MIRROR").unwrap_or_else(|_| default_base.into());
   let version = env::var("CARGO_PKG_VERSION").unwrap();
   let target = env::var("TARGET").unwrap();
   if cfg!(target_os = "windows") {

[piscisaureus]

@picoHz I'm open to adding this feature. Can you send the proposed change in the form of a PR?

[piscisaureus]

#446