Build grimblast from source for Arch-based systems

[hurlbutt]

Description

#1783 discusses removing AUR-dependencies if possible. Right now, Arch-users using this project's dotfiles will install grimblast (used for screenshots) from the AUR while other distros build directly from source. Given we already have the logic in place to install grimblast fresh for all other distros, we can extend this to grimblast as well.

Changes

• Removed grimblast-git as an Arch-distro dependency
• [X} Updated post-arch.sh to query pacman for grimblast-git, and if not found download and build grimblast from source.

How Has This Been Tested?

Hasn't been tested end-to-end, but will post in #1783 to see if reporter can assist with testing.

• Tested on Arch Linux/Based Distro.
• Tested on Fedora Linux/Based Distro.
• Tested on openSuse.

Checklist

Please ensure your pull request meets the following requirements:

• My code follows the style guidelines of this project.
• I have performed a self-review of my code.
• I have commented on my code, particularly in hard-to-understand areas.
• My changes do not introduce new warnings.
• I have added tests that prove my fix is effective or that my feature works.
• I have made corresponding changes to the documentation
• New and existing unit tests pass locally with my changes.

Related Issues

#1783

[hurlbutt]

#1792 provides most of these changes, so I modified the PR to just check for grimblast-git being installed/managed before attempting to install from source.