The --legacy-tarball SCons option is a vestige of the period where we were transitioning to hygienic builds. We can do without the flag now, and we should remove it.

The flag exists to synthesize a special value for $PREFIX when we are doing a build for release.

We should consider instead whether we can achieve something better by restructuring the various DIST variables, so that evergreen itself can be in charge of describing the intended installation prefix.