Add an environment variable to skip cross build check in build.rs

[secext2022]

Cross build deno is possible with some special method (#1846, #19759).

Currently we have to patch build.rs like this:

diff '--color=auto' -ru -x Cargo.lock workdir-1/deno-1.36.2/build.rs workdir/deno-1.36.2/build.rs
--- workdir-1/deno-1.36.2/build.rs	2023-08-24 19:38:40.831557143 +0800
+++ workdir/deno-1.36.2/build.rs	2023-08-24 19:39:43.532680484 +0800
@@ -400,9 +400,6 @@
   // Host snapshots won't work when cross compiling.
   let target = env::var("TARGET").unwrap();
   let host = env::var("HOST").unwrap();
-  if target != host {
-    panic!("Cross compiling with snapshot is not supported.");
-  }
 
   let symbols_path = std::path::Path::new("napi").join(
     format!("generated_symbol_exports_list_{}.def", env::consts::OS).as_str(),

But if we add an environment variable like this:

diff '--color=auto' -ru -x Cargo.lock workdir-1/deno-1.36.2/build.rs workdir/deno-1.36.2/build.rs
--- workdir-1/deno-1.36.2/build.rs	2023-08-24 19:38:40.831557143 +0800
+++ workdir/deno-1.36.2/build.rs	2023-08-25 14:02:31.747904547 +0800
@@ -400,7 +400,8 @@
   // Host snapshots won't work when cross compiling.
   let target = env::var("TARGET").unwrap();
   let host = env::var("HOST").unwrap();
-  if target != host {
+  let skip_cross_build = env::var("DENO_SKIP_CROSS_BUILD_CHECK").map_or(false, |v| v == "1");
+  if (!skip_cross_build) && (target != host) {
     panic!("Cross compiling with snapshot is not supported.");
   }

Then when do cross build, we can just export DENO_SKIP_CROSS_BUILD_CHECK=1, and no need to patch build.rs.

But when some one do cross build accidentally (not setup special method for cross build), they will still get warned:

  thread 'main' panicked at 'Cross compiling with snapshot is not supported.', build.rs:405:5

[mmastrac]

@secext2022 Can you turn this into a PR? I'll merge it and we can try to get aarch64 builds for Linux going.

[secext2022]

@mmastrac

#20533

[secext2022]

This issue should close, because the PR merged.