CI: github: merge Test and Test-musl in VTest.yml

CI: github: merge Test and Test-musl in VTest.yml

In commit 7640d794 ("CI: Integrate Musl build into vtest.yml"), the
alpine job was integrated into VTest.yml. However, most of the task are
still duplicated and changes in the workflow need edits of copy/paste
code in two places because of that.

This commit deduplicates the code by making the alpine job part of the
matrix, like it was done for macOS.