Write manifest for MAJOR.MINOR channel to enable rustup convenience

This connects to rust-lang/rustup#794.

It's hard to remember if there have been patch releases for old versions
when you'd like to install the latest in a MAJOR.MINOR series.

When we're doing a stable release, we write duplicate manifests to
`stable`. With this change, only when we're doing a stable release, also
write duplicate manifests to `MAJOR.MINOR` to eventually enable rustup
(and any other tooling that builds Rust release URLs) to request, say,
`1.45` and get `1.45.2` (assuming `1.45.2` is the latest available
`1.45` and assuming that we never publish patch releases out of order).

src/tools/build-manifest/src/main.rs

@@ -293,6 +293,8 @@ impl Builder {
         self.write_channel_files(self.versions.channel(), &manifest);
         if self.versions.channel() != rust_version {
             self.write_channel_files(&rust_version, &manifest);
+            let major_minor = rust_version.split('.').take(2).collect::<Vec<_>>().join(".");
+            self.write_channel_files(&major_minor, &manifest);
         }
     }