unixPB: Fix CentOS6 SCL repo baseurl

[Willsparker]

Fixes: #1745 (hopefully)
Ref: #1778 , #1765

Changes the baseurl of the CentOS-SCL and CentOS-SCL-rh repo. I've used win_shell and sed, as opposed to replace / lineinfile, primarily because I couldn't get those modules' regex to work. The linter may not like that ...

• commit message has one of the standard prefixes
• playbook changes run through VPC or QPC Yep! :https://ci.adoptopenjdk.net/job/VagrantPlaybookCheck/977/

Note, the above check may uncovered other issues with CentOS6, but it does fix the SCL repo issue.

[sxa]

LGTM. This will help some other stuff I'm doing. Looks like the build in the VPC has pretty much completed as well so we can merge this as soon as it's completed.