CI: use purism arm64 image

3 jobs for librem5 in 39 minutes and 52 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  .Pre
canceled #298082
flatpak
flatpak

00:01:18

passed #239206
librem5:arm64:priv
flatpak-arm64

00:38:34

passed #239205
flatpak
flatpak

00:06:48