ci: add alpine build to github ci

Problem: There is no automated build of flux-core on a musl
libc system, so PRs are not checked for portability to non-glibc
environments.

Add a ci build on alpine to the github workflow matrix. Only run
`make check-prep` since there are still many fixes required for
sharness tests in this environment.

src/test/generate-matrix.py

@@ -270,6 +270,12 @@ def __str__(self):
     docker_tag=True,
 )
 
+matrix.add_build(
+    name="alpine - unit tests only",
+    image="alpine",
+    command_args="--check-prep",
+)
+
 # inception
 matrix.add_build(
     name="inception",