package/go: bump version to 1.12.8

package/go: bump version to 1.12.8

go1.12.6 (released 2019/06/11) includes fixes to the compiler, the linker, the
go command, and the crypto/x509, net/http, and os packages.

go1.12.7 (released 2019/07/08) includes fixes to cgo, the compiler, and the
linker.

go1.12.8 (released 2019/08/13) includes security fixes to the net/http and
net/url packages.

https://golang.org/doc/devel/release.html

Signed-off-by: Christian Stewart <christian@paral.in>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>