We added CentOS 7's /etc/pki/ca-trust/extracted/pem/tls-ca-bundle.pem
to the list in response to #17549 - not being able to find any certs otherwise.

Now we have #18813, where CentOS 6 apparently has both that file
and /etc/pki/tls/certs/ca-bundle.crt, and the latter is complete while
the former is not.

Moving the new CentOS 7 file to the bottom of the list should fix both
problems: the CentOS 7 system that didn't have any of the other files
in the list will still find the new one, and existing systems will still
keep using what they were using instead of preferring the new path
that may or may not be complete on some systems.

Fixes #18813.

Change-Id: I5275ab67424b95e7210e14938d3e986c8caee0ba
Reviewed-on: https://go-review.googlesource.com/36429
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Adam Langley <agl@golang.org>