[RFC] Enable usage of rust-sccache for cargo builds

[icp1994]

Reviving #30702 - I tested the changes in this PR: YES

I have been using this for almost a week and it has reduced build times for rust packages quite considerably in my laptop. I have addressed some of the questions/suggestions from the reviews of the previous PR.

Although not the most scientific benchmark, I noted the build times from the cargo output while packaging 4 templates across iterations. In the first two iterations, all of them ran without sccache with an older version of the template to get a baseline without the interference of the crates being downloaded. Third iteration was with same old versions but with sccache enabled which created the cache of all 4 packages combined. Finally, last iteration with the full cache and the latest version of the templates - a typical scenario while updating a package. The difference between the second and the fourth iterations are as follows:

rust-analyzer | 6m 13s | 4m 49s
gping         | 1m 44s | 40.81s
difftastic    | 1m 32s | 34.80s
streampager   | 1m 14s | 23.25s

I will add docs for README later if there is consensus to merge this eventually.

[Duncaen]

I really don't like implicit build-style dependencies, especially with logic like this.

[icp1994]

I agree it's not the cleanest. But isn't it explicit since XBPS_SCCACHE is opt-in?

[icp1994]

With ppc support being dropped from void, don't need the arch checking logic - so this is as "clean" as it's going to be.

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[Vinfall]

Does this make sccache co-exist with ccache?

I'm using ccache globally inside & outside of void-packages with different cache directories and would like to enable sccache for cargo builds only (or better, Rust codes only, so I can re-use existing Fortran/C/C++ cache in ccache). Is this possible?

[icp1994]

Yes,it only modifies the Rust build helper and stores it's stuff at /hostdir/sccache

[Vinfall]

That makes a lot of sense, I'm a bit hesitated to adopt sccache globally due to hit rate but it sounds reasonable to try it in packaging.

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