mockchain: Preserve failing buildroots by default
mockchain: Preserve failing buildroots by default

Sometimes I've wanted to be able to dive into the root and debug why
something failed.  Ideally mock would support this "preserve *failing*
buildroots" as a builtin option, but we can implement it outside
easily enough.

stahnma/rpmdistro-gitoverlay,stahnma/rpmdistro-gitoverlay,maxamillion/rpmdistro-gitoverlay,maxamillion/rpmdistro-gitoverlay