ENV/std: port superenv SDK changes

[Bo98]

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Now that the superenv SDK changes have stabilised, let's incorporate those changes into the stdenv.

This, along with the minor change to test.rb, means that the test environment is now consistent with the build environment in terms of SDK selection. This fixes a couple of errors caused by that inconsistency.

[MikeMcQuaid]

Looks good! Some style suggestions. Some may be wrong because I find unless ... || hard to parse but: I'm sure you get the idea.