package/go: bump version to 1.12.4

package/go: bump version to 1.12.4

Fixes a number of issues discovered since 1.12.1.  From the release notes:

go1.12.2 (released 2019/04/05) includes fixes to the compiler, the go
command, the runtime, and the doc, net, net/http/httputil, and os packages.
See the Go 1.12.2 milestone on our issue tracker for details.

go1.12.3 (released 2019/04/08) was accidentally released without its
intended fix.  It is identical to go1.12.2, except for its version number.
The intended fix is in go1.12.4.

go1.12.4 (released 2019/04/11) fixes an issue where using the prebuilt
binary releases on older versions of GNU/Linux led to failures when linking
programs that used cgo.  Only Linux users who hit this issue need to update.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>