Using flatpak-builder-args breaks Flatter

[PhrozenByte]

One can use neither the suggested --default-branch=nightly, nor --skip-if-unchanged options of flatpak-builder (see README.md) using the flatpak-builder-args option:

• --default-branch=nightly causes flatpak build-bundle to fail later, because it apparently still assumes master as branch name
• --skip-if-unchanged causes flatpak-builder to exit with code 42 (as documented by flatpak-builder), being interpreted as build failure by Flatter