build-kmod-kit: don't rebuild if kernel hasn't changed

[tjkirch]

If the kernel archive is older than the kmod kit, we can assume that the kmod
kit is up to date and skip its 20+ second build.

Note: This is built on top of #1285. The goal is to have this be fast enough that it can be added as a dependency to build-variant, so the kmod kit can consistently be added to repos.

Testing done:

With no existing archive, a new one is prepared:

> ls build/archives/
> cargo make build-archives
...
[cargo-make] INFO - Running Task: build-packages
    Finished dev [optimized] target(s) in 0.12s
[cargo-make] INFO - Running Task: build-kmod-kit
[cargo-make] INFO - Build Done in 24 seconds.

A rerun notices the existing artifact:

[cargo-make] INFO - Running Task: build-kmod-kit
Existing kmod kit /home/tjk/work/bottlerocket/build/archives/aws-k8s-1.17-x86_64-kmod-kit-v1.0.5.tar.xz is newer than kernel archive /home/tjk/work/bottlerocket/build/rpms/bottlerocket-x86_64-kernel-archive-5.4.80-1.x86_64.rpm; skipping build.
[cargo-make] INFO - Build Done in 3 seconds.

Updating the kernel results in a new archive being build:

> touch build/rpms/*kernel*
> cargo make build-archives
...
[cargo-make] INFO - Running Task: build-kmod-kit
[cargo-make] INFO - Build Done in 24 seconds.

Removing the kernel forces the new error. (This wouldn't normally happen because build-kmod-kit depends on build-packages which builds this kernel archive.)

> mv build/rpms/*kernel* ~
> cargo make build-archives
...
[cargo-make] INFO - Running Task: build-kmod-kit
Unable to find latest kernel archive for x86_64 in /home/tjk/work/bottlerocket/build/rpms
[cargo-make] ERROR - Error while executing command, exit code: 1

Terms of contribution:

By submitting this pull request, I agree that this contribution is dual-licensed under the terms of both the Apache License, version 2.0, and the MIT license.