novnc: use installed package files as default for `--web`

[dwf]

Description of changes

novnc in nixpkgs right now complains:

$ novnc --vnc localhost:5901
Could not find vnc.html

The files are installed in ${pkgs.novnc}/share/webapps/novnc. This patches utils/novnc_proxy to supply this as a default for the argument to --web, as well as the help text.

Things done

• Built on platform(s)
  • [x] x86_64-linux
  • [ ] aarch64-linux
  • [ ] x86_64-darwin
  • [ ] aarch64-darwin
• [ ] For non-Linux: Is sandbox = true set in nix.conf? (See Nix manual)
• [ ] Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• [x] Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• [ ] Tested basic functionality of all binary files (usually in ./result/bin/)
• 22.11 Release Notes (or backporting 22.05 Release notes)
  • [ ] (Package updates) Added a release notes entry if the change is major or breaking
  • [ ] (Module updates) Added a release notes entry if the change is significant
  • [ ] (Module addition) Added a release notes entry if adding a new NixOS module
  • [ ] (Release notes changes) Ran nixos/doc/manual/md-to-db.sh to update generated release notes
• [x] Fits CONTRIBUTING.md.

[dwf]

@AndersonTorres You approved #153540 which added novnc, you might be an appropriate second reviewer for this PR, which is a quality of life improvement over the current packaging.

I've realized that this is technically a breaking change, I think, as if someone is relying on the behaviour of searching the CWD, it will fail them. Should I add to the release notes?

[NeverBehave]

@AndersonTorres You approved #153540 which added novnc, you might be an appropriate second reviewer for this PR, which is a quality of life improvement over the current packaging.

I've realized that this is technically a breaking change, I think, as if someone is relying on the behaviour of searching the CWD, it will fail them. Should I add to the release notes?

Maybe, but I don't think it really make sense unless it is for some development purposes. We use this package mainly for its static content and serving thru nginx. That's why I didn't bother with that behavior at first. FYI

[dwf]

@applePrincess Mind giving this a look? (For some reason I cannot "request review")

[dwf]

@AndersonTorres I switched to using substituteAllInPlace, thank you for the suggestion. Please take another look when you have a chance.