This almost never matters, but if the path-global 'rake' or 'rails'
points to a specific (and wrong) ruby version, (or, possible in CI,
there is no installed 'rails' executable), things get confused.
Instead, any time we mean "run a global 'rails', as for 'new'", use a
fully-qualified path to our in-tree copy. And any time we're working
inside an application, use the bin/rails script directly. It would be
equivalently valid to always use the one in exe/, because that handles
searching for bin/rails internally... but it's uglier to fully-qualify,
plus 'rake' would then be more complicated.
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