tools/customvet: consider $GO environment variable

For backports it's usually convenient to be able to set the $GO
environment variable to the version used in the particular stable
branch, e.g. on v1.10 we'd set GO=go1.16.15. However, that setting is
currently not considered in tools/customvet. This can lead to unexpected
failures such as the followin when running `GO=go1.16.15 make precheck`
on v1.10 branch:

    contrib/scripts/custom-vet-check.sh
    main: internal error: package "sync" without types was imported from "github.com/cilium/cilium/pkg/cleanup"
    exit status 1

Signed-off-by: Tobias Klauser <tobias@cilium.io>

tools/customvet

@@ -1,4 +1,4 @@
 #!/usr/bin/env bash
 SCRIPTPATH="$( cd "$(dirname "$0")" ; pwd -P )"
 
-go run ${SCRIPTPATH}/../vendor/github.com/cilium/customvet/main.go $@
+${GO:-go} run ${SCRIPTPATH}/../vendor/github.com/cilium/customvet/main.go $@