The cover variable indices could vary from build to build,
but they were not included in the build ID hash, so that
reusing the previously built package was not safe.
Make the indices no longer vary from build to build,
so that caching is safe.
Fixes #22652.
Change-Id: Ie26d73c648aadd285f97e0bf39619cabc3da54f2
Reviewed-on: https://go-review.googlesource.com/81515
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Ian Lance Taylor <iant@golang.org>