plex-desktop: remove version from desktop exec

[detroyejr]

In #345754 we removed the version from the name in order to get a reliable name. However, I neglected to remove the version from the exec string of the plex-desktop.desktop file. Didn't think to check this since the executable in result/bin was working as expected.

Sorry for multiple PR's on the same issue.

Things done

Tested with:

# Build
nix-build -A plex-desktop

# "Install" the application with nix-env.
nix-env -i ./result

Programs like rofi now launch the app correctly where previously they tried to use the old command format with the version appended (e.g. plex-desktop-1.110).

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• 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/)
• 24.11 Release Notes (or backporting 23.11 and 24.05 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
• Fits CONTRIBUTING.md.

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