pve-novnc: support update script

[codgician]

This PR attempts to make pve-novnc support the existing update script by:

• Using Proxmox's git source and clone the original novnc as submodule
• Specify sourceRoot to ./novnc to make existing build scripts compatible
• Add a --version-prefix parameter to update.py so we can limit the major.minor version of update target version. This is done to make sure build commands in nixpkgs are compatible for automatically updated versions.

Welcome any discussions on this method of working-around auto updates. If this is an acceptable solution for similar packages that needs both src and src_patches (e.g. pve-qemu), I plan to invest some time in updating these packages using the same pattern of this PR.

Tested by manually setting it to an older commit, then:

nix-shell tasks/update.nix --argstr package pve-novnc