internal/testenv: reduce init-time work for MustHaveExec
In CL 486275 I added a somewhat complex init function that sets up a
callback to probe for exec support. A lot of the complexity was simply
to avoid an unnecessary call to os.Environ during init.
In CL 491660, I made the os.Environ call unconditional on all
platforms anyway in order to make HasGoBuild more robust.
Since the init-function indirection no longer serves a useful purpose,
I would like to simplify it to a package-level function, avoiding the
complexity of changing package variables at init time.