Store build artifacts per architecture

[tjkirch]

Description of changes:

Previously, switching between architectures would rebuild all artifacts, which
can make building and testing changes painful.

This change makes cargo and buildsys store their state in arch-specific
directories, so when you switch, they can find the last artifacts they built
for that arch.

Note because it originally confused me: we don't need to change anything about the RPM builds or the Rust/cargo used inside package builds, because the cache mount used in package builds is already arch-specific.

Testing done:

There's a relatively high blast radius for relatively few lines of code, so let me know if you can think of other tests to run.

• Build x86_64 aws-k8s-1.20 from scratch. Confirm health.
• Check build/state/; here's an example of a marker for a package and for a variant artifact.

   662017      0 -rw-r--r--   1 tjk      tjk             0 Jun 24 09:56 x86_64/packages/grep/bottlerocket-x86_64-grep-debugsource-3.6-1.x86_64.rpm.buildsys_marker
   923063      0 -rw-r--r--   1 tjk      tjk             0 Jun 24 10:10 x86_64/variants/aws-k8s-1.20/bottlerocket-aws-k8s-1.20-x86_64-1.1.2-18c6be33-root.verity.lz4.buildsys_marker

• Build x86_64 again. Artifacts are reused, build is fast. (Only variant artifact state is changed, no packages.) Confirm health.
• Build aarch64 from scratch. Confirm health.
• Check build/state; x86 state isn't touched. Example ARM markers:

   923418      0 -rw-r--r--   1 tjk      tjk             0 Jun 24 10:20 aarch64/packages/grep/bottlerocket-aarch64-grep-debuginfo-3.6-1.x86_64.rpm.buildsys_marker
  1054430      0 -rw-r--r--   1 tjk      tjk             0 Jun 24 10:26 aarch64/variants/aws-k8s-1.20/bottlerocket-aws-k8s-1.20-aarch64-1.1.2-18c6be33.img.lz4.buildsys_marker

• Build aarch64 again; build is fast. Confirm health.
• Build x86_64 again; build is fast. aarch64 state isn't touched, only x86 variant artifacts. Confirm health.
• Build aarch64 again; build is fast. x86 state isn't touched, only aarch64 variant artifacts. Confirm health.
• Build aws-ecs-1 variant for x86 just to confirm nothing funny is going on between variants. It builds the ECS-specific packages and updates their state markers, reuses the rest. Confirm health.
• Touch storewolf/src/main.rs, build x86_64, confirm it rebuilds os, build aarch64, confirm it rebuilds os again.

The (top-level) cargo fingerprint/cache is separated correctly; example package:

drwxr-xr-x. 2 tjk tjk 4.0K 06-24 10:18 variants/target/aarch64/debug/.fingerprint/acpid-eb8670ac655d56a0/
drwxr-xr-x. 2 tjk tjk 4.0K 06-24 09:54 variants/target/x86_64/debug/.fingerprint/acpid-eb8670ac655d56a0/

(Before, it was like that, but minus the $arch level.)

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.

[webern]

nice