fix: make rust-chromium a separate implementation

[thomaseizinger]

The current caching implementation expects only a single docker container per implementation directory. This was not true for Rust v0.52 which produced two containers: A native one and one with chromium and a WASM build.

Thus, the latter never got cached and we were rebuilding it on every run. By making it a separate implementation, the cache should be picked up again.

I refrained from extending the cache implementation to handle this because I think it would get too complicated.

Resolves: #301.

[thomaseizinger]

Marking this as draft until I can confirm that it actually works.

[thomaseizinger]

It should work now:

• Here is where the cache for the chromium build would be loaded: https://github.com/libp2p/test-plans/actions/runs/6170406724/job/16746635420?pr=302#step:3:2485
• Hereis where the cache for the native build would be loaded: https://github.com/libp2p/test-plans/actions/runs/6170406724/job/16746635420?pr=302#step:3:734

We can see that the two keys are different.

[mxinden]

Sounds good to me! Thank you for debugging.