Desktop: Fix desktop services not starting after wlr-randr v0.5.0

[kajusnau]

Description of changes

1. Fixed Launcher and Taskbar Services Not Starting After wlr-randr v0.5.0:
   • wlr-randr v0.5.0 rejects standalone CLI arguments, preventing ghaf-launcher and ewwbar services from starting.
   • Forced taskbar and launcher services to use Bash as an interpreter in Exec commands to ensure proper execution when redirecting output.
   • General note for devs: Systemd units do not use shell as an interpreter for Exec commands.
     It's therefore recommended to always call a script with no arguments nor options, or use bash -c (as shown in this PR) to interpret commands properly.

Checklist for things done

• Summary of the proposed changes in the PR description
• More detailed description in the commit message(s)
• Commits are squashed into relevant entities - avoid a lot of minimal dev time commits in the PR
• Contribution guidelines followed
• Ghaf documentation updated with the commit - https://tiiuae.github.io/ghaf/
• PR linked to architecture documentation and requirement(s) (ticket id)
• Test procedure described (or includes tests). Select one or more:
  • Tested on Lenovo X1 x86_64
  • Tested on Jetson Orin NX or AGX aarch64
  • Tested on Polarfire riscv64
• Author has run make-checks and it passes
• All automatic Github Action checks pass - see actions
• Author has added reviewers and removed PR draft status
• Change requires full re-installation
• Change can be updated with nixos-rebuild ... switch

Instructions for Testing

• List all targets that this applies to:
• Is this a new feature
  • List the test steps to verify:
    • Verify fix works as expected, taskbar and ghaf launcher work at boot.
    • Verify previously introduced fix for log flooding in headless mode still works and is effective.
• If it is an improvement how does it impact existing functionality?

[milva-unikie]

Tested on Lenovo-X1 (nixos-rebuild switch):

• Taskbar and app menu are back.
• Test automation passes.

Also checked the pre-merge pipeline results to confirm that Orin-NX log size remains reduced.