reverse build/host activation order, to give build exe's higher priority

[msarahan]

@mingwandroid reported problems building R recipes where he needed the compilers to be in the host section for some reason. This PR is a WIP as we explore this idea - it might break stuff pretty badly.

[mingwandroid]

he needed the compilers to be in the host section for some reason

This is because R users expect to be able to call install.packages() from within the REPL and for things involving compilation that requires compilers (much the same as on Python with sysconfig).

I want our R infrastructure (and MRO) on Linux to use our compilers rather than reporting an error or building broken software.

(and to clarify, the compilers are not in the host section through their own 'volition', they are there transitively due to r-base or mro-base being in the host section).

[github-actions]

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