package/go: bump version to 1.13.12

package/go: bump version to 1.13.12

go1.13.9 (released 2020/03/19) includes fixes to the go command, tools, the
runtime, the toolchain, and the crypto/cypher package.

go1.13.10 (released 2020/04/08) includes fixes to the go command, the runtime,
and the os/exec and time packages.

go1.13.11 (released 2020/05/14) includes fixes to the compiler.

go1.13.12 (released 2020/06/01) includes fixes to the runtime, and the go/types
and math/big packages.

Release notes: https://golang.org/doc/go1.13

Signed-off-by: Christian Stewart <christian@paral.in>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>