contrib/scripts: consider $GO in check-fmt.sh
contrib/scripts: consider $GO in check-fmt.sh

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 contrib/scripts/check-fmt.sh. If there were
changes in how gofmt formats code between Go versions and the system's
Go version is newer than the $GO setting, using that gofmt binary might
lead to check-fmt failure.

Signed-off-by: Tobias Klauser <059b8b880f8441509ec8a65b50b4c6ae74ebea76@cilium.io>

cilium/cilium,cilium/cilium,cilium/cilium,cilium/cilium,cilium/cilium