gnu: guix: Install SysV init files to $(prefix)/etc.

Fixes <https://bugs.gnu.org/40153>.
Reported by Maxim Cournoyer <maxim.cournoyer@gmail.com>.

This is a followup to 73fbe041.

* gnu/packages/package-management.scm (guix)[arguments]: In 'bootstrap'
phase, change "sysvinitservicedir" in 'nix/local.mk'.