[projects] Use correct type for environment var.

Set an environment variable to string, not symbol.
[environment] Expose target platform to environment.

This exposes the target platform setting to the environment via
show target-platform and set target-platform in the dylan-compiler
command line UI.
Propagate changes to the build-script when target-platform changes.
Merge pull request #818 from waywardmonkeys/target-platform-property

Target platform property