release/dist: switch back to Ubuntu 20.04 for building QNAP packages

After the switch to 24.04, unsigned packages did not build correctly (came out as only a few KBs).

Fixes tailscale/tailscale-qpkg#148

Signed-off-by: Percy Wegmann <percy@tailscale.com>

release/dist/qnap/files/scripts/Dockerfile.qpkg

@@ -1,4 +1,4 @@
-FROM ubuntu:24.04
+FROM ubuntu:20.04
 
 RUN apt-get update -y && \
     apt-get install -y --no-install-recommends \