Also updated scripts/update_golang_version.sh to handle new CircleCI
file layout.

Despite being a security release for Go, we do not believe Nomad is
impacted.

1.15.5 is due in a couple days but we may release before then.

Upgrade to golang 1.15

Starting with golang 1.5, setting Ctty value result in `Setctty set but Ctty not valid in child` error, as part of https://github.com/golang/go/issues/29458 .
This commit lifts the fix in https://github.com/creack/pty/pull/97 .

Go 1.14.7 fixes CVE-2020-16845 which is not believed to impact Nomad.

Pick up fixes [golang 1.14.6 bug fixes](https://github.com/golang/go/issues?q=milestone%3AGo1.14.6+label%3ACherryPickApproved), specially the one where reflect.DeepEqual returns true even if values don't match, affecting our tests integrity.

Go 1.14.4 contains two CVEs which are fixed in 1.14.5:

 - [CVE-2020-15586](https://golang.org/issue/34902)
 - [CVE-2020-14039](https://golang.org/issue/39360)

Upon consideration with HashiCorp security these CVEs are considered low
severity for Nomad and no new security fix binary will be released.

Use golang 1.12.12

Use use base official golang image in CircleCI, as it gets refreshed
more quickly compared to circleci/golang, and we don't benefit from
circleci image customizations much.

To get fix for https://github.com/golang/go/issues/34540 .

It's the latest 1.11 point release as of now

To correspond to the Linux scripts in 855761a5