mantle/platform/qemu: fix seting of LIBGUESTFS env vars

This setting of cmd.Env was overriding the LIBGUESTFS_BACKEND=direct
that was being set just a few lines before. So the command was getting
run with just LIBGUESTFS_HV being set, which happens to have no
effect unless LIBGUESTFS_BACKEND=direct is set.

Fix that by appending to cmd.Env on the second modification.

mantle/platform/qemu.go

@@ -660,7 +660,7 @@ func newGuestfish(diskImagePath string, diskSectorSize int) (*coreosGuestfish, e
 	cmd.Env = append(os.Environ(), "LIBGUESTFS_BACKEND=direct")
 	switch system.RpmArch() {
 	case "ppc64le":
-		cmd.Env = append(os.Environ(), "LIBGUESTFS_HV=/usr/lib/coreos-assembler/libguestfs-ppc64le-wrapper.sh")
+		cmd.Env = append(cmd.Env, "LIBGUESTFS_HV=/usr/lib/coreos-assembler/libguestfs-ppc64le-wrapper.sh")
 	}
 	// make sure it inherits stderr so we see any error message
 	cmd.Stderr = os.Stderr