Revise CircleCI caching strategies

[tlaurion]

Context:

• Clean build without cache on nix docker image: build time 1h6 minutes https://app.circleci.com/pipelines/github/tlaurion/heads/2538/workflows/7e6f69f6-7568-4edb-b3cf-a5c59fdf94ef
• Build with cache on nix docker image (rebuild to reuse cache): build time 1h8 minutes https://app.circleci.com/pipelines/github/tlaurion/heads/2538/workflows/c8d82efa-aa49-48b5-bf53-cffd1c22268b

Cache speeding up builds : fail.

• Time to download cache layer: 20 minutes https://app.circleci.com/pipelines/github/tlaurion/heads/2538/workflows/c8d82efa-aa49-48b5-bf53-cffd1c22268b/jobs/45679
• Attaching a workspace prior of build : 2-3 minutes https://app.circleci.com/pipelines/github/tlaurion/heads/2538/workflows/c8d82efa-aa49-48b5-bf53-cffd1c22268b/jobs/45682/parallel-runs/0/steps/0-101
• Persisting build to next board being dependent: 5 minutes https://app.circleci.com/pipelines/github/tlaurion/heads/2538/workflows/c8d82efa-aa49-48b5-bf53-cffd1c22268b/jobs/45682/parallel-runs/0/steps/0-108
• Save cache (useless since same cache on rebuild) : 13 minutes https://app.circleci.com/pipelines/github/tlaurion/heads/2538/workflows/c8d82efa-aa49-48b5-bf53-cffd1c22268b/jobs/45708

Basically we loose around 45 minutes, some of which blocks other boards from building.

Some thoughts:

• We should build muslcross alone on circleci and chache and reuse that.
• For the rest i'm not sure I see the point of using caching anymore for to save CPU time from CI at the cost of more bandwidth and waiting time prior of starting builds in most use case
• On nix, we could put musl-cross-make as first step for both x86 and ppc64 under docker image and modify Makefile to check if provided per nix and if the case use it
• advance on prior step and use nix coreboot-buildstacks

After that, optimizations would come from moving all modules into nix expressions. But I'm not sure we want to go there yet. Seems to close contributions to nix developers which I do not categorize myself in still.