Run the tests as a separate flake

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

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