Change default var override from 'projects and subprojects' to amalgamation

The 'projects and subprojects' semantics resulted in some counter-intuitive
behavior. For example, in a project with tests/ as a subproject if one builds
one of the tests directly with a non-global override (say C++ compiler), then
the main project would be built without the overrides. I this light,
overriding in the whole amalgamation seems like the right thing to do. The
old behavior can still be obtained with scope qualification, for example:

b ./:foo=bar

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