runtime: Use _srt_find_myself() to locate pressure-vessel prefix
This is slightly simpler, and makes it easy for PvRuntime to locate tools in the helpers path (libexec/steam-runtime-tools-0) too.
Edited by Simon McVittie
This is slightly simpler, and makes it easy for PvRuntime to locate tools in the helpers path (libexec/steam-runtime-tools-0) too.