nixos/calibre-web: fix malformed environment variable

nixos/modules/services/web-apps/calibre-web.nix

···
         after = [ "network.target" ];
         wantedBy = [ "multi-user.target" ];
 
+        # fix book cover cache directory defaults to a path under /nix/store/
+        environment.CACHE_DIR = "/var/cache/calibre-web";
+
         serviceConfig = {
           Type = "simple";
           User = cfg.user;
···
 
           CacheDirectory = "calibre-web";
           CacheDirectoryMode = "0750";
-          environment.CACHE_DIR = "/var/cache/calibre-web";
         }
         // lib.optionalAttrs (!(lib.hasPrefix "/" cfg.dataDir)) {
           StateDirectory = cfg.dataDir;