Run the tests as a separate flake

Avoids needing the `--impure` flag, and hopefully makes everything more
nicely cached

.github/workflows/test.yml

@@ -25,11 +25,8 @@ jobs:
     - run: nix-build -A checks.$(nix-instantiate --eval -E '(builtins.currentSystem)')
     - run: |
         cachix use nixos-nix-install-tests
-        NIX_BUILD=$(nix-build -A defaultPackage.$(nix-instantiate --eval -E '(builtins.currentSystem)'))/bin/nix-build
-        $NIX_BUILD \
-          --option experimental-features 'flakes' \
-          --option access-tokens github.com=${{ secrets.GITHUB_TOKEN }} \
-          --impure --expr '(builtins.getFlake (builtins.toPath ./.)).lib.testAgainst.${builtins.currentSystem} (builtins.getFlake "github:nixos/nix/master").defaultPackage.${builtins.currentSystem}'
+        CURRENT_NIX_BUILD=$(nix-build -A defaultPackage.$(nix-instantiate --eval -E '(builtins.currentSystem)'))/bin
+        PATH=$CURRENT_NIX_BUILD:$PATH bash scripts/test-against-master.sh
   check_cachix:
     name: Cachix secret present for installer tests
     runs-on: ubuntu-latest

scripts/test-against-master.sh

@@ -0,0 +1,27 @@
+#!/usr/bin/env bash
+
+set -euo pipefail
+set -x;
+
+CurrentNixDir=$(pwd)
+CurrentRev=${GITHUB_SHA:-$(git rev-parse HEAD)}
+WorkDir=$(mktemp -d)
+trap 'rm -r "$WorkDir"' EXIT
+
+pushd "$WorkDir"
+
+cat <<EOF > flake.nix
+{
+    inputs.currentNix.url = "git+file://$CurrentNixDir?rev=$CurrentRev";
+    inputs.nixMaster.url = "github:nixos/nix";
+
+    outputs = { self, currentNix, nixMaster }: {
+        checks = builtins.mapAttrs (systemName: _:
+        { againstMaster = currentNix.lib.testAgainst.\${systemName} nixMaster.defaultPackage.\${systemName}; }
+        ) currentNix.defaultPackage;
+    };
+}
+EOF
+nix --experimental-features 'nix-command flakes' flake check
+
+popd