app-emulation/containerd: set GOFLAGS to -mod=vendor

Since containerd 1.5 started to turn on Go module, we need to pass
`-mod=vendor` to the go build command.
Otherwise, go build will fail because it would try to fetch missing
go deps from remote repos. It would not work inside of sandbox.

We cannot set `COREOS_GO_MOD=vendor` because containerd ebuild calls
`emake` instead of `go_build`.

sdk_container/src/third_party/coreos-overlay/app-emulation/containerd/containerd-9999.ebuild

@@ -54,6 +54,8 @@ src_prepare() {
 src_compile() {
 	local options=( $(usex btrfs "" "no_btrfs") )
 	export GOPATH="${WORKDIR}/${P}" # ${PWD}/vendor
+	export GO111MODULE=on
+	export GOFLAGS="-v -x -mod=vendor"
 	LDFLAGS=$(usex hardened '-extldflags -fno-PIC' '') emake BUILDTAGS="${options[*]}"
 }