cinnamon: Fix settings theme thumbnail path + Fix crash with virtual keyboard in screensaver

[bobby285271]

Description of changes

cinnamon.cinnamon-common: Fix settings theme thumbnail path:

• We look for globally installed themes with this path, so we use
  /run/current-system/sw/share.

cinnamon.cinnamon-screensaver: Fix crash with virtual keyboard:

• Caribou.KeyboardModel is also used here, so we apply the same fix for
  cinnamon-common to get rid of a crash.

/cc #101881

Things done

• Built on platform(s)
  • 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
• 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/)
• 23.05 Release Notes (or backporting 22.11 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
• Fits CONTRIBUTING.md.

[bobby285271]

(nixosTests.cinnamon does not build on master currently, this PR will fix that)
@ofborg test cinnamon