nspawn idmaps the store on erofs, which the VM test's store uses; the
allowlist from the store check refused it and failed the namespaced-agent
subtest in vm-nightly #7084. vm-nightly #7030 ran the same agent on the
same erofs store green. The magic is EROFS_SUPER_MAGIC_V1 from the
kernel's linux/magic.h; libc has no constant.
Refs #5888