go build -o <your desired name>
You can specify the executable name using the -o switch with go build. For your example it would look something like:
cd $GOPATH/github.com/username/go-foobar && go build -o foobar. However, you’re just left with the executable in the package’s folder — you still need to install it somehow.
However, I don’t know of any way to specify that for someone using go get github.com/username/go-foobar to install your tool. For instance, see this answer: https://stackoverflow.com/a/33243591/2415176
If you’re not worried about people installing your tool with go get, this is the kind of thing you can wrap in a Makefile.