Revert "test: use btrfs by default on Arch as well"

Revert "test: use btrfs by default on Arch as well"

There's something very wrong going on when using btrfs for the test
images, namely:
  - there's a significant performance hit, i.e. the Arch Linux run is
    ~20% slower, in the coverage run the situation is even worse
  - intermittent boot failures
  - intermittent "No space left on device" errors (even though there's
    enough free space)

Since debugging this might take a while, let's temporarily revert back
to ext4 to make the CI stable again.

This reverts commit 7eb7e3ec4f5dbc13ee729557e1544527f3101187.