swift234

[build-script] Add infrastructure for passing host-specific variables.

 - This isn't yet used, but we need an easy way for `build-script` to
   pass host-specific variables down to `build-script-impl`.

 - I don't think it is worth complicating the main argument parsing
   logic with a syntax for passing associative arrays on the command
   line, so this just passes them via environment variables.

 - Part of SR-237.