Upgrade Golang versions in custom Docker build

1. go1.13 -> go1.13.4
2. go1.12.5 -> go1.12.13

The 1.12 upgrade matches what we are using for Omnibus:
https://gitlab.com/gitlab-org/gitlab-omnibus-builder/merge_requests/125

scripts/custom-docker-build

@@ -19,12 +19,12 @@ function print_golang_args() {
             GOLANG_DOWNLOAD_SHA256=aefaa228b68641e266d1f23f1d95dba33f17552ba132878b65bb798ffa37e6d0
             ;;
         1.12)
-            INSTALL_GOLANG_VERSION=1.12.5
-            GOLANG_DOWNLOAD_SHA256=aea86e3c73495f205929cfebba0d63f1382c8ac59be081b6351681415f4063cf
+            INSTALL_GOLANG_VERSION=1.12.13
+            GOLANG_DOWNLOAD_SHA256=da036454cb3353f9f507f0ceed4048feac611065e4e1818b434365eb32ac9bdc
             ;;
         1.13)
-            INSTALL_GOLANG_VERSION=1.13
-            GOLANG_DOWNLOAD_SHA256=68a2297eb099d1a76097905a2ce334e3155004ec08cdea85f24527be3c48e856
+            INSTALL_GOLANG_VERSION=1.13.4
+            GOLANG_DOWNLOAD_SHA256=692d17071736f74be04a72a06dab9cac1cd759377bd85316e52b2227604c004c
             ;;
         *) echo "Unknown golang version $1"; exit 1;
     esac