tests.nixpkgs-check-by-name: Fix with parallel tests

We seem to have enough tests to run into this now:

    error: creating symlink from '/private/tmp/nix-build-nixpkgs-check-by-name.drv-0/source/test-tmp/var/nix/gcroots/profiles' to '/private/tmp/nix-build-nixpkgs-check-by-name.drv-0/source/test-tmp/var/nix/profiles': File exists

pkgs/test/nixpkgs-check-by-name/default.nix

@@ -25,6 +25,10 @@ let
         export NIX_LOG_DIR=$TEST_ROOT/var/log/nix
         export NIX_STATE_DIR=$TEST_ROOT/var/nix
         export NIX_STORE_DIR=$TEST_ROOT/store
+
+        # cargo tests run in parallel by default, which would then run into
+        # https://github.com/NixOS/nix/issues/2706 unless the store is initialised first
+        nix-store --init
       '';
       postCheck = ''
         cargo fmt --check