arm64: builds failing because boot partition is too small

I have noticed that testing and unstable builds for some arm64 images - pine64-lts and pine64-plus have been failing while generating the initramfs. The following error can be seen in debootstrap.log.

1292   Setting up initramfs-tools (0.142) ...
1293   update-initramfs: deferring update (trigger activated)
1294   Processing triggers for initramfs-tools (0.142) ...
1295   update-initramfs: Generating /boot/initrd.img-6.1.0-6-arm64
1296   W: No zstd in /usr/bin:/sbin:/bin, using gzip
1297    
1298   gzip: stdout: No space left on device
1299   E: mkinitramfs failure gzip 1

I was able to build pine64 images successfully by setting boot_size = '192MiB' in builders/arm.py