Makefile: introduce GO_BUILD variable

[tklauser]

This allows users to overwrite the GO environment variable while still
using CGO_ENABLED=0. This for example is useful when testing
pre-releases of the Go toolchain, e.g. make GO=go1.17rc1.

Follows cilium/hubble#560

[tklauser]

• GKE workflow failed with

 The connection to the server 34.94.161.100 was refused - did you specify the right host or port?

while waiting for the cilium-cli job to complete: https://github.com/cilium/cilium-cli/actions/runs/1029446401

• External Workloads workflow failed with

 error: timed out waiting for the condition on jobs/cilium-cli-install

while waiting for the cilium-cli job to complete and

Error from server: Get "https://10.168.0.111:10250/containerLogs/kube-system/cilium-cli-install-77pwm/test?timestamps=true": No SSH tunnels currently open. Were the targets able to accept an ssh-key for user "gke-be2e2552123e4033aa0e"?

while retrieving post-installation logs: https://github.com/cilium/cilium-cli/actions/runs/1029446395

Could these be caused by infrastructure issues on GKE, given both run there? Re-triggering these workflows.