RHEL 7 introduces a new tool, update-ca-trust(8), which places the
certificate bundle in a new location. Add this path to the list of
locations that are searched for the certificate bundle.

Fixes #15749

Change-Id: Idc97f885ee48ef085f1eb4dacbd1c2cf55f94ff5
Reviewed-on: https://go-review.googlesource.com/30375Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>