alpine: use root to avoid nosuid issue in ci

problem: just like with el9 and noble, alpine requires user to be set to
root in the dockerfile here to build right under qemu-user-static in CI

solution: add the user root line where required

src/test/docker/alpine/Dockerfile

@@ -2,6 +2,7 @@ FROM fluxrm/flux-core:alpine
 
 ARG USER=flux
 ARG UID=1000
+USER root
 
 # Install extra buildrequires for flux-sched:
 RUN sudo apk add \