Fix Makefile by using first element of GOPATH
Fix Makefile by using first element of GOPATH

If GOPATH includes multiple paths, running `make promu` would fail.
This fixes it by always using the first path.

chosenken/node_exporter,siavashs/node_exporter,bs-github/node_exporter,siavashs/node_exporter,derekmarcotte/node_exporter,bs-github/node_exporter,ema/node_exporter,joehandzik/node_exporter,prometheus/node_exporter,knweiss/node_exporter,joehandzik/node_exporter,joehandzik/node_exporter,joehandzik/node_exporter,chosenken/node_exporter,derekmarcotte/node_exporter,ema/node_exporter,bs-github/node_exporter,knweiss/node_exporter,knweiss/node_exporter,derekmarcotte/node_exporter,knweiss/node_exporter,ema/node_exporter,prometheus/node_exporter,bs-github/node_exporter,derekmarcotte/node_exporter,ema/node_exporter,Jean-Daniel/node_exporter,chosenken/node_exporter,siavashs/node_exporter,Jean-Daniel/node_exporter,chosenken/node_exporter,Jean-Daniel/node_exporter,siavashs/node_exporter,prometheus/node_exporter,Jean-Daniel/node_exporter