Stop pre-building Go std library

[bboreham]

These lines: https://github.com/weaveworks/build-tools/blob/dedde7fa74541639a8223f15260ee71f1a13ee6a/build/golang/Dockerfile#L25-L27

are not necessary since Go added a per-target build cache, and probably don't achieve anything since we typically bind-mount another directory for the build cache when compilations are run: https://github.com/weaveworks/prom-aggregation-gateway/blob/c4415bbe1411d77eb4132f3df51c87cf7fcca840/Makefile#L60