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.

Change-Id: Ie0041d52cbde06ff14540192c8fba869a851158e
Reviewed-on: https://go-review.googlesource.com/c/go/+/492977
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

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