Reassess musl testing strategy

Bicep uses separate CI jobs to run CLI tests against a Linux musl system. We started doing this because the `mcr.microsoft.com/azure-cli` image, based on Alpine Linux, was commonly used by Bicep users. As of version 2.64.0, however, the image has migrated to Mariner (Azure Linux), a glibc distro.

For the moment, we've pinned the image used by the `linux-musl-x64` CI jobs at `mcr.microsoft.com/azure-cli:2.63.0`, but we should reassess whether this testing is something we still need to do now that the `azure-cli` image is no longer based on a musl distro, and, if so, what a better long-term strategy would be.

Start by inspecting the CI definitions for the linux-musl-x64 jobs and their pinned mcr.microsoft.com/azure-cli:2.63.0 image. Compare the current test coverage with the image's Mariner-based environment and determine whether musl testing remains necessary; done means an agreed long-term testing strategy and corresponding CI changes.

Written by the indexing model from the issue text.