0.15.1: a vcpkg installation waits for another one of the same root - #33

Under 0.15.0, GalTranslPP (Windows) failed in mcpp build -p GPPCLI. Two workspace members install one vcpkg root, and their actions ran at once:

vcpkg-running.lock: error: failed to take lock, another vcpkg may be running against the same directory

0.13.0 and 0.14.0 serialised the two installations with their own lock, inside mcpp-deps. Since 0.15.0 the action is vcpkg itself, and vcpkg waits for its root lock only when asked to. The action now passes --x-wait-for-lock.

Measured locally with two concurrent vcpkg install runs on one root: with the flag, both exit 0 (one reports "waiting to take filesystem lock"); without it, one exits 1. The vcpkg-workspace criterion now also requires the flag in both members' plans. Local runs of vcpkg-consumer and vcpkg-workspace pass. The docs statement that the lock alone serialised the installations is corrected.