Change BuildEnv to a map

[peterebden]

Decided while working on #3143 that this was messy as a slice. Heaps of code needs to loop over it and do string splitting and whatnot, and keeping on having to worry about duplicating entries which this now just does naturally.

This seems a lot neater. Pretty sure I've caught everywhere that uses it but there's the odd use that can slip through (e.g. iterating the return value of a function without naming the type might still compile).

It's hopefully more efficient, although I doubt it shows up much (whenever we need to construct env vars, we're executing a subprocess which will dominate total time).