crypto/x509: support RHEL 7 cert bundle
crypto/x509: support RHEL 7 cert bundle

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/30375
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

goodlang/good,goodlang/good,ImJasonH/go,derekmarcotte/go,codestation/go,vsekhar/elastic-go,suncycheng/go,ImJasonH/go,goodlang/good,cloudflare/go,josharian/go,suncycheng/go,momchil-velikov/go,josharian/go,nawawi/go,akutz/go,CAFxX/go,josharian/go,CAFxX/go,akutz/go,golang/go,momchil-velikov/go,vsekhar/elastic-go,polyverse-security/go,nawawi/go,codestation/go,golang/go,cloudflare/go,momchil-velikov/go,vsekhar/elastic-go,polyverse-security/go,cloudflare/go,codestation/go,suncycheng/go,linux-on-ibm-z/go,akutz/go,nawawi/go,Cofyc/go,CAFxX/go,josharian/go,ImJasonH/go,CAFxX/go,golang/go,derekmarcotte/go,goodlang/good,goodlang/good,polyverse-security/go,linux-on-ibm-z/go,momchil-velikov/go,vsekhar/elastic-go,nawawi/go,josharian/go,suncycheng/go,codestation/go,akutz/go,suncycheng/go,ImJasonH/go,linux-on-ibm-z/go,nawawi/go,golang/go,Cofyc/go,suncycheng/go,suncycheng/go,nawawi/go,cloudflare/go,goodlang/good,vsekhar/elastic-go,akutz/go,codestation/go,codestation/go,Cofyc/go,cloudflare/go,linux-on-ibm-z/go,polyverse-security/go,derekmarcotte/go,polyverse-security/go,momchil-velikov/go,vsekhar/elastic-go,momchil-velikov/go,derekmarcotte/go,golang/go,nawawi/go,Cofyc/go,linux-on-ibm-z/go,ImJasonH/go,CAFxX/go,golang/go,momchil-velikov/go,akutz/go,golang/go,CAFxX/go,cloudflare/go,derekmarcotte/go,polyverse-security/go,linux-on-ibm-z/go,ImJasonH/go,vsekhar/elastic-go,vsekhar/elastic-go,CAFxX/go,nawawi/go,akutz/go,linux-on-ibm-z/go,golang/go,suncycheng/go,josharian/go,goodlang/good,CAFxX/go,christopher-henderson/Go,cloudflare/go,derekmarcotte/go,Cofyc/go,codestation/go,goodlang/good,cloudflare/go,momchil-velikov/go,Cofyc/go,derekmarcotte/go,derekmarcotte/go,Cofyc/go,ImJasonH/go,josharian/go,ImJasonH/go,linux-on-ibm-z/go,Cofyc/go,codestation/go,polyverse-security/go,akutz/go,polyverse-security/go,josharian/go