Use --reflink=always when copying the fallback initramfs

It saves us ~20MB.
6 jobs for cp-reflink in 29 minutes and 11 seconds (queued for 2 seconds)
Status Name Job ID Coverage
  Lint
passed shellcheck #14703

00:00:18

passed shfmt #14704

00:00:09

 
  Build
passed build #14708
fast-single-thread

00:25:14

failed build #14705
fast-single-thread

00:00:48

 
  Test
passed test-cloudimg-qemu #14707

00:03:37

passed test-vagrant-boxes-format #14706

00:00:33