Enable RedHat 6 in coreclr master (#13315)

Enable RedHat 6 in coreclr master (#13315)

* Enable RedHat 6 in coreclr master

Enable RedHat 6 in coreclr master.

This is identical as the approved PR to enable RedHat 6 in coreclr
release/2.0.0:
https://github.com/dotnet/coreclr/pull/13301

* Correct Rid to match /src/.nuget/dirs.props

Correct Rid to match /src/.nuget/dirs.props

* Update dockertag and add logic to detect RHEL6 in init-tools.sh

Update dockertag and add logic to detect RHEL6 in init-tools.sh

* Port changes from Release/2.0.0 to fix coreclr RHEL 6 official runs.

Port changes from Release/2.0.0 to fix coreclr RHEL 6 official runs.

* Remove empty quotes.

Remove empty quotes.