CI: add missing dependency as reported on appveyor

Error message reads:

Unpacking singularity-ce (4.1.0-focal) ...
dpkg: dependency problems prevent configuration of singularity-ce:
 singularity-ce depends on fuse2fs; however:
  Package fuse2fs is not installed.
dpkg: error processing package singularity-ce (--install):

.appveyor.yml

@@ -26,7 +26,7 @@ shallow_clone: false
 environment:
   DTS: datalad_next
   APPVEYOR_BUILD_WORKER_IMAGE: Ubuntu2004
-  INSTALL_SYSPKGS: graphicsmagick-imagemagick-compat moreutils jq uidmap
+  INSTALL_SYSPKGS: graphicsmagick-imagemagick-compat moreutils jq uidmap fuse2fs
   # go with whatever is most recent to catch anything that might break
   # with them
   INSTALL_GITANNEX: git-annex -m snapshot