ExternalProject: Restore default log dir with custom stamp dir
In !2488 (merged) the log directory got its own option. The intention was to fall back to the stamp directory by default. However, the implementation actually only falls back to the same default as the stamp directory and does not consider a custom stamp dir.
Update the default log dir computation to fall back to whatever is the final selection for the stamp dir.
Fixes: #19000 (closed)