nspawn: replace strextend_with_separator() -> strextendf_with_separator()

Fixes a bug introduced by cfea7618.

Before this commit:
mode=1777,size=10%,nr_inodes=400k,uid=496107520,gid=496107520,context=,sys.id:sys.role:systemd.nspawn.container.fs:s0,
After this commit:
mode=1777,size=10%,nr_inodes=400k,uid=496107520,gid=496107520,context=sys.id:sys.role:systemd.nspawn.container.fs:s0

Fixes #19976.