Promote "alternative" rustc builds to Nightly archives

Nightly builds are archived under `https://static.rust-lang.org/dist/{date}/`.
This adds a new `alt/` subdirectory containing:

* `rustc-nightly-x86_64-apple-darwin.tar.gz`
* `rustc-nightly-x86_64-unknown-linux-gnu.tar.gz`
* `rustc-nightly-x86_64-pc-windows-msvc.tar.gz`

copied from `s3://rust-lang-ci2/rustc-builds-alt/`.
These are alternative builds of rustc with LLVM assertions disabled,
which reduces testing coverage but improves compiler performance.

This adds these resources to those that are considered “stable and public”
per https://internals.rust-lang.org/t/public-stable-rust-services/6072.
For the latest Nightly they take up 143 MiB.
At current S3 pricing, this increases the cost of storing 10 years worth
of Nightlies by 14 USD per month.

This fixes rust-lang/rust#45334
and will enable Servo’s build system to stop relying
on private and ephemeral resources.

promote-release/src/main.rs

@@ -147,7 +147,7 @@ impl Context {
             let file = t!(file);
             t!(fs::copy(file.path(), self.dl_dir().join(file.file_name())));
         }
-        self.publish_archive();
+        self.publish_archive(rev);
         self.publish_docs();
         self.publish_release();
 
@@ -311,7 +311,7 @@ upload-addr = \"{}/{}\"
                 .arg(&dst));
     }
 
-    fn publish_archive(&mut self) {
+    fn publish_archive(&mut self, rev: &str) {
         let bucket = self.secrets["dist"]["upload-bucket"].as_str().unwrap();
         let dir = self.secrets["dist"]["upload-dir"].as_str().unwrap();
         let dst = format!("s3://{}/{}/{}/", bucket, dir, self.date);
@@ -321,6 +321,19 @@ upload-addr = \"{}/{}\"
                 .arg("--only-show-errors")
                 .arg(format!("{}/", self.dl_dir().display()))
                 .arg(&dst));
+        if self.release == "nightly" {
+            let src = format!("s3://rust-lang-ci2/rustc-builds-alt/{}/", rev);
+            let dst = format!("s3://{}/{}/{}/alt/", bucket, dir, self.date);
+            run(self.aws_s3()
+                    .arg("cp")
+                    .arg("--recursive")
+                    .arg("--only-show-errors")
+                    .arg("--exclude").arg("*")
+                    .arg("--include").arg("rustc-*.tar.gz")
+                    .arg("--exclude").arg("rustc-*src.tar.gz")
+                    .arg(src)
+                    .arg(dst));
+        }
     }
 
     fn publish_docs(&mut self) {