Run tests on pull requests

Pull requests will automatically trigger a build and test run. The
produced binaries are stored as artifacts in the Github workflow.

Makefile

@@ -55,8 +55,8 @@ README-options:
 	rm options.txt.tmp
 
 tools:
-	go get github.com/mitchellh/gox
-	go get github.com/tcnksm/ghr
+	go install github.com/mitchellh/gox@latest
+	go install github.com/tcnksm/ghr@latest
 
 test:
 	if [ `go fmt $(go list ./... | grep -v /vendor/) | wc -l` -gt 0 ]; then echo "go fmt error"; exit 1; fi