Testing on three different OS X machines (including the master), I see this in the build console output:

Unpacking Go from http://golang.org/dl/go1.3.darwin-amd64-osx10.6.tar.gz

However, these machines all have 10.9.3, so should install the "osx10.8" package.

System properties on the tested slaves are identical:

os.arch     x86_64
os.name	    Mac OS X
os.version  10.9.3