Chore: Use stable alpine for RISC-V

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What does this MR do?

Depends on !5730 (merged).

Followup for !4226 (merged). Previously, RISC-V used the alpine:edge image by default, unlike all other architectures. Initially, this was because Alpine only had RISC-V support in edge. Alpine's RISC-V support is stable now, so there's no need to default to the unstable edge image anymore.

Considering !4226 (merged) only introduced edge, maybe it could make sense to completely drop builds of the edge helper images again. I'd appreciate a decision on that from the GitLab team, because it would technically be a breaking change.

Why was this MR needed?

Using the unstable edge image by default is normally a good idea.

What's the best way to test this MR?

Use the GitLab runner on RISC-V and check the edge image is no longer used by default.

What are the relevant issue numbers?