fix(nixery): use set-default for setting WEB_DIR

Makes it possible for users to still override this using an envvar.

Relates to nixery#166

Change-Id: Ief2925e03cf2e4351bc38554bf553c8ee259f1f7
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11045
Autosubmit: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI
Reviewed-by: flokli <flokli@flokli.de>

default.nix

@@ -59,7 +59,7 @@ depot.nix.readTree.drvTargets rec {
     nativeBuildInputs = [ makeWrapper ];
     postInstall = ''
       wrapProgram $out/bin/server \
-        --set WEB_DIR "${nixery-web}" \
+        --set-default WEB_DIR "${nixery-web}" \
         --prefix PATH : ${nixery-prepare-image}/bin
     '';