CI, build 3: Stop sharding cache on containerd version

[apostasie]

For context, see #3940, #3924, and the start of this PR cycle at #3983.

On top of #3984

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We currently shard our cache per-containerd version.

There is more downsides than upsides in doing this. Specifically:

• it does multiply our cache use by as many versions of containerd we are testing (currently three, soon two)
• because we are more likely to hit GHA limits, this triggers cascading cache evictions, forcing unnecessary full rebuilds and further increasing communication with the cache backend, leading to throttling

While not sharding and running in parallel will indeed drop some layers that could be cached until the cache is fully warm, this should be out-weighted by the benefits.

[apostasie]

EL8 failure is TestStopWithStopSignal - see #3979

[apostasie]

Failure is #3513

[AkihiroSuda]

Thanks