update pipeline to support 1.12 now that it is out

[zachgersh]

Seems like the pipeline dynamic-metalink resource is going to need a bit of an update since 1.12 shipped.

cc @dpb587-pivotal

[dpb587]

Might be required to configure GOCACHE when adding 1.12 support (requested in #12). I saw the following error from my binary's packaging script when it was trying to autoupgrade go (patch).

build cache is required, but could not be located: GOCACHE is not defined and neither $XDG_CACHE_HOME nor $HOME are defined

[edwardecook]

We'd be keen to see this added to golang-release as soon as possible as we are planning to bump to Golang 1.12 this month if possible. We would happily contribute although we don't know the process.

cc @jimbo459

[kieron-dev]

Services Enablement are also keen to see 1.12 added here. We would like to release soon with go-1.12 so would like an idea of if / when 1.12 will be added, so we can manually add a go-1.12 tarball if necessary.

[mrosecrance]

1.12 is now added. For future reference, everything is pipelined so the work is largely a find and replace as shown here (there was also some GoCache stuff). I'm waiting on getting added to this repo to close this issue.