ci(flatpak): only build for x86_64

The flatpaks from CI aren't very usable in the first place, but also
take longer to complete than regular builds, as well as contribute to
our concurrent job limit. Dropping ARM builds shouldn't have much
impact, but this can obviously be reversed if people want it

Signed-off-by: Seth Flynn <getchoo@tuta.io>

.github/workflows/flatpak.yml

@@ -77,9 +77,6 @@ jobs:
           - os: ubuntu-22.04
             arch: x86_64
 
-          - os: ubuntu-22.04-arm
-            arch: aarch64
-
     runs-on: ${{ matrix.os }}
 
     container: