🌱 make install was not installing to GOPATH

[ristomcgehee]

• Please check if the PR fulfills these requirements

• PR title follows the guidelines defined in https://github.com/ossf/scorecard/blob/main/CONTRIBUTING.md#pr-process

• What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)
  Tooling bug fix

• What is the current behavior? (You can also link to an open issue here)
  I have my GOPATH set to a different location:

~/code/ossf/scorecard(main)$ echo $GOPATH
/home/chris/.go

The line

GOPATH := $(go env GOPATH)

appears to be overwriting my environment variable and turning it into a local variable. If I modify the install target to:

.PHONY: install
install: ## Installs all dependencies needed to compile Scorecard
install: | $(PROTOC)
	@echo Installing tools from tools/tools.go
	echo $$GOPATH
	go env GOPATH
	cd tools; cat tools.go | grep _ | awk -F'"' '{print $$2}' | xargs -tI % go install %

It outputs:

~/code/ossf/scorecard(main)$ make install
Installing tools from tools/tools.go
echo $GOPATH

go env GOPATH
/home/chris/go
cd tools; cat tools.go | grep _ | awk -F'"' '{print $2}' | xargs -tI % go install %
...

This causes the tools to be installed in /home/chris/go which is not in my $PATH.

• What is the new behavior (if this is a feature change)?
  I checked with Naveen who added this line in 🌱 Move tool dependencies into go.mod #460, and he thought it would be fine to remove this line.

• Does this PR introduce a breaking change? (What changes might users need to make in their application due to this PR?)
  No

[naveensrinivasan]

Thanks @chrismcgehee!