Setting qbs.installRoot in a product can fail under certain circumstances

This happens because the executor always chooses the install root of whatever happens to be the "first" product as the value to pass to the product installer.

So it can happen that every product that actually installs something, sets qbs.installRoot to the same value, while the "first" product, which might not even install anything, gets its install root chosen and everything gets installed there.

This is important for Qbs because we want to set the qbs.installRoot to different values in different product multiplex instances so that we can combine multiple install roots into a single multi-architecture package.

Currently, the engine chooses the value of (presumably) the "version" product, which simply lists the VERSION file in the project and does not set any qbs.installRoot value, and ignores the values passed to the actually relevant products.