ports-mgmt/sccache-overlay: increase prio to fix cargo at configure time
- increases the priority for the start-server to fix a race with "250:cargo-configure" (fixes a configure time error for py-granian and others)
- add a start-safeguard (helps when running "poudriere testpost -i" during debugging) to not start it multiple times
PR: 296177
Maintainer timeout: 2 weeks
Fixes: 292835