Build stable image paths using symlinks in build/latest/

[tjkirch]

Issue number:

Fixes #725

Description of changes:

This adds stable paths for the most recently built artifacts of each arch/variant combination. Stable paths make our builds easier for newcomers to understand, make the setup experience easier to document, and make it easier to do informal scripting around build processes. The links are in a new directory to reduce the chance of using them accidentally.

Testing done:

I built aws-k8s and saw the links in place:

$ ll build/latest
total 80K
drwxr-xr-x 2 tjk domain^users 4.0K 02-21 12:13 ./
drwxr-xr-x 3 tjk domain^users  52K 02-21 12:13 ../
lrwxrwxrwx 1 tjk domain^users   65 02-21 12:13 bottlerocket-x86_64-aws-k8s-boot.ext4.lz4 -> ../bottlerocket-x86_64-aws-k8s-0.2.2-a1d831a5-dirty-boot.ext4.lz4
lrwxrwxrwx 1 tjk domain^users   64 02-21 12:13 bottlerocket-x86_64-aws-k8s-data.img.lz4 -> ../bottlerocket-x86_64-aws-k8s-0.2.2-a1d831a5-dirty-data.img.lz4
lrwxrwxrwx 1 tjk domain^users   59 02-21 12:13 bottlerocket-x86_64-aws-k8s.img.lz4 -> ../bottlerocket-x86_64-aws-k8s-0.2.2-a1d831a5-dirty.img.lz4
lrwxrwxrwx 1 tjk domain^users   66 02-21 12:13 bottlerocket-x86_64-aws-k8s-migrations.tar -> ../bottlerocket-x86_64-aws-k8s-0.2.2-a1d831a5-dirty-migrations.tar
lrwxrwxrwx 1 tjk domain^users   65 02-21 12:13 bottlerocket-x86_64-aws-k8s-root.ext4.lz4 -> ../bottlerocket-x86_64-aws-k8s-0.2.2-a1d831a5-dirty-root.ext4.lz4
lrwxrwxrwx 1 tjk domain^users   67 02-21 12:13 bottlerocket-x86_64-aws-k8s-root.verity.lz4 -> ../bottlerocket-x86_64-aws-k8s-0.2.2-a1d831a5-dirty-root.verity.lz4

I rebuilt aws-k8s and saw the links disappear during the build, then reappear at the end.

I built aws-dev and saw that the aws-k8s links were not touched, and aws-dev links were created at the end.

[tjkirch]

This push makes cargo make clean remove the latest directory, too, since it's removing all built images.