installer: simplify the per-build installation

The goal is to simplify the installation and testing of arbitrary Nix
versions.

1. Extend the base installer to accept a NIX_RELEASE_URL to change where
   Nix is getting installed from.
2. Generate a new output from Hydra that combines all the released
   tarballs.

Once this is merged it should allow to:
1. Pick an evaluation at https://hydra.nixos.org/jobset/nix/master that
   looks healthy
2. Select the installedScript build and get the "install" download link.
3. Select the allTarballs build and get the download link.

Now equipped with all of this:

    curl -sfL https://hydra.nixos.org/build/1234/download/1/install | \
      NIX_RELEASE_URL=https://hydra.nixos.org/build/5678/download/1/XXX sh

Fixes NixOS#4047

flake.nix

@@ -311,6 +311,18 @@
                 $(cat ${installerClosureInfo}/store-paths)
             '');
 
+        # Group all the tarballs into a single symlink tree
+        allTarballs =
+          with nixpkgsFor.x86_64-linux;
+          runCommand "all-tarballs" {}
+          ''
+            mkdir $out/nix-${version}
+            for output in ${builtins.attrValues self.hydraJobs.binaryTarball};
+            do
+              ln -s "$output"/*.tar.xz "$out/nix-${version}"
+            end
+          '';
+
         # The first half of the installation script. This is uploaded
         # to https://nixos.org/nix/install. It downloads the binary
         # tarball for the user's system and calls the second half of the

scripts/install.in

@@ -32,7 +32,11 @@ case "$(uname -s).$(uname -m)" in
     *) oops "sorry, there is no binary distribution of Nix for your platform";;
 esac
 
-url="https://releases.nixos.org/nix/nix-@nixVersion@/nix-@nixVersion@-$system.tar.xz"
+# Set this environment variable to switch where Nix will fetch the tarball
+# from. Since the content is verified after download, changing this is still
+# secure.
+: "${NIX_RELEASE_URL:-https://releases.nixos.org/nix}"
+url="${NIX_RELEASE_URL}/nix-@nixVersion@/nix-@nixVersion@-$system.tar.xz"
 
 tarball="$tmpDir/$(basename "$tmpDir/nix-@nixVersion@-$system.tar.xz")"