tests/functional: Add flake-based regression for debugger use-after-free

This is the simplest reproducer I have. It would be great to find
a repro without flakes, but I guess this should be ok for now.

tests/functional/flakes/debugger.sh

@@ -0,0 +1,28 @@
+#!/usr/bin/env bash
+
+source ./common.sh
+
+requireGit
+
+flakeDir="$TEST_ROOT/flake"
+createGitRepo "$flakeDir"
+
+cat >"$flakeDir/flake.nix" <<EOF
+{
+  inputs = {
+  };
+
+  outputs =
+    _:
+    let
+    in
+    {
+      packages.$system.default = throw "oh no";
+    };
+}
+EOF
+
+git -C "$flakeDir" add flake.nix
+
+# regression #12527 and #11286
+echo ":env" | expect 1 nix eval "$flakeDir#packages.${system}.default" --debugger