installerScriptForGHA: aarch64-darwin

GitHub Actions seems to have magically switched architectures
without changing their identifiers.
See https://github.com/actions/runner-images/blob/2813ee66cbe85b31a8322ff8967148548b1a5db9/README.md#available-images
Maybe they have more complete documentation elsewhere, but it
seems to be incapable of selecting a runner based on architecture.

(cherry picked from commit df3e92f)

build/hydra.nix

@@ -98,7 +98,7 @@ in
   installerScriptForGHA = installScriptFor [
     # Native
     self.hydraJobs.binaryTarball."x86_64-linux"
-    self.hydraJobs.binaryTarball."x86_64-darwin"
+    self.hydraJobs.binaryTarball."aarch64-darwin"
     # Cross
     self.hydraJobs.binaryTarballCross."x86_64-linux"."armv6l-unknown-linux-gnueabihf"
     self.hydraJobs.binaryTarballCross."x86_64-linux"."armv7l-unknown-linux-gnueabihf"