Changes

debian-trixie-obs-latest: fix signed-by= path

Fix for:
  Error: Failed to parse keyring "/usr/share/keyrings/osmocom-misc.asc"

Fixes: 59d469 ("debian-trixie-build: install the osmocom:misc repo")
Change-Id: Ib90abcdf96fc2b555f477899bac730c6f4a76fad