Currently, EnvironmentVariablesObject is a strange
holder-that's-not-a-holder. This has implicaitons for things that expect
to get an EnvironmentVariables object, as we can't automatically
unholder it, and instead have to to manually do so. Now we can
automatically unholder it, which makes everything much nicer.
It should build the fallback subprject with default_library=static and
override the dependency for both static=True and static kwarg not given.
Fixes: #8050.