Add godot-version-string output derived from version.py

Callers that install export templates need the version directory name
(e.g. 4.6.4.rc or 4.6.0.stable) to install to the right path. Previously
they had to hardcode it or re-derive it from the source tree themselves.
This output is always populated after the locate-bins step regardless of
which targets were built.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

v1 — Windows clang default (working SCons cache, ~2h->~8min); cross-p…

…latform integration tests