Linux build fixes related to AppImage

• Removed a preprocessor conditional in KisAppimageUpdater so that KisAppimageUpdaterTest won't fail to build when git hash cannot be obtained.
• Some fixes to the AppImage build script:
  • [[ -d .git ]] -> git rev-parse --is-inside-work-tree for when using git-worktree (.git is a special text file instead of a dir)
  • Fix various non-functional code
  • [ is_beta -eq 0 -o is_rc -eq 0 ] -> [ $is_beta -eq 0 ] || [ $is_rc -eq 0 ] as according to shellcheck, Prefer [ p ] || [ q ] as [ p -o q ] is not well defined.

CC: @amedonosova

Test Plan

Run a docker AppImage build without the .git dir.

Formalities Checklist

• I confirmed this builds.
• I confirmed Krita ran and the relevant functions work.
• I tested the relevant unit tests and can confirm they are not broken. (If not possible, don't hesitate to ask for help!)
• I made sure my commits build individually and have good descriptions as per KDE guidelines.
• I made sure my code conforms to the standards set in the HACKING file.
• I can confirm the code is licensed and attributed appropriately, and that unattributed code is mine, as per KDE Licensing Policy.