Make sure CFG_RELEASE_CHANNEL is always set.

Previously we'd build libsyntax without it when documenting and that'd
cause us to recompile it when building normally for no reason.

src/bootstrap/builder.rs

@@ -426,6 +426,7 @@ pub fn rustdoc_cmd(&self, compiler: Compiler) -> Command {
                 self.sysroot(compiler)
             })
             .env("RUSTC_LIBDIR", self.sysroot_libdir(compiler, self.build.build))
+            .env("CFG_RELEASE_CHANNEL", &self.build.config.channel)
             .env("RUSTDOC_REAL", self.rustdoc(compiler));
         cmd
     }
@@ -574,6 +575,9 @@ pub fn cargo(&self,
         // FIXME: should update code to not require this env var
         cargo.env("CFG_COMPILER_HOST_TRIPLE", target);
 
+        // Set this for all builds to make sure doc builds also get it.
+        cargo.env("CFG_RELEASE_CHANNEL", &self.build.config.channel);
+
         if self.is_verbose() {
             cargo.arg("-v");
         }