[ci/mac] Upgrade to macOS 10.12 for build workers.
[ci/mac] Upgrade to macOS 10.12 for build workers.

This change will cause all builds of `core/hab` on macOS to use version
10.12. Prior to this change 10.11 was used but this infrastructure was
retired within the last couple of weeks.

Signed-off-by: Fletcher Nichol <77a0fd9e8048bbacd11af4e957bc6ff03b549f49@nichol.ca>

rsertelon/habitat,habitat-sh/habitat,rsertelon/habitat,rsertelon/habitat,habitat-sh/habitat,rsertelon/habitat,rsertelon/habitat,rsertelon/habitat,habitat-sh/habitat,habitat-sh/habitat,habitat-sh/habitat,habitat-sh/habitat,habitat-sh/habitat,rsertelon/habitat,habitat-sh/habitat