add flatpak builder cache exclude preset

[renner0e]

This adds a preset to exclude the .flatpak-builder directory on linux.

Description

when you build flatpaks with flatpak-builder there is gonna be directory that contains libraries, cached files, compiled binaries etc. on a per project basis. This is obviously data that is relatively useless to backup.
This is gonna exclude every directory called .flatpak-builder anywhere on the filesystem.

Motivation and Context

it can get really big in size
for vorta it is like 400MiB
for some rust gui apps for example a couple of gigs

How Has This Been Tested?

I forked the repo made the necessary changes and built the flatpak from master branch and ran a backup of a git folder that I knew has a .flatpak-builder directory and it is not there in the archive.

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• I have read the CONTRIBUTING guide.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

I provide my contribution under the terms of the license of this repository and I affirm the Developer Certificate of Origin.

[shivansh02]

Thanks for the PR! Looks good to me.