diff --git a/pkgs/testing/flake-module.nix b/pkgs/testing/flake-module.nix
index 918fb5d05..539199148 100644
--- a/pkgs/testing/flake-module.nix
+++ b/pkgs/testing/flake-module.nix
@@ -3,6 +3,7 @@
     legacyPackages.setupNixInNix = ''
       export HOME=$TMPDIR
       export NIX_STATE_DIR=$TMPDIR/nix
+      export NIX_CONF_DIR=$TMPDIR/etc
       export IN_NIX_SANDBOX=1
       export CLAN_TEST_STORE=$TMPDIR/store
       # required to prevent concurrent 'nix flake lock' operations