Update Makefile and GitHub action to stop relying on GOPATH

[radu-matei]

#47 moved the dependency model used by Signy from dep to Go modules, meaning we don't need to build the project in GOPATH anymore.

However, there are a few places in Makefile where we assume the system that is building the project has set a value for the GOPATH environment variable, for example:

signy/Makefile

Line 42 in 21e031d

curl -sfL https://install.goreleaser.com/github.com/golangci/golangci-lint.sh | sh -s -- -b $(GOPATH)/bin $(GOLANGCI_VERSION)

Note that there is also a go get in the bootstrap target which (I think?) still needs a valid GOBIN location.
We should update the install and bootstrap targets to stop assuming one, and update the GitHub action so we don't set up all the environment variables:

signy/.github/workflows/ci.yml

Line 21 in 21e031d

export GOPATH=$HOME/go && export GOBIN=$(go env GOPATH)/bin && export PATH=$PATH:$GOPATH&& export PATH=$PATH:$GOBIN && mkdir -p $GOBIN