Makefile: don't run duplicate cmd in TESTPKGS_EVAL
Makefile: don't run duplicate cmd in TESTPKGS_EVAL

Previously, `TESTPKGS_EVAL` would shell out to `$(GO_LIST)` when that
had already been executed. This commit uses the output of the previous
command instead of running the command twice.

Signed-off-by: Chris Tarazi <711c73f64afdce07b7e38039a96d2224209e9a6c@isovalent.com>

cilium/cilium,tklauser/cilium,michi-covalent/cilium,tklauser/cilium,cilium/cilium,tgraf/cilium,michi-covalent/cilium,tklauser/cilium,michi-covalent/cilium,cilium/cilium,cilium/cilium,tklauser/cilium,tgraf/cilium,cilium-team/cilium,cilium/cilium,michi-covalent/cilium,tgraf/cilium,cilium-team/cilium,tklauser/cilium,michi-covalent/cilium,tgraf/cilium,tgraf/cilium,tgraf/cilium