build: configure subproject sysprof with -Dagent=false
Sysprof has a new -Dagent=true build option which allows installing a /usr/bin/sysprof-agent program (simimlar to sysprof-cli). It provides a P2P D-Bus API to the process which can control subprocesses. It's used by IDE tooling to have more control across container boundaries.
However, we do not need it for GTK CI.