Do a brew update, rather than brew install, if Go is already inst…

…alled.

Summary: Either way, we try to pin them to go1.12, for consistency.

Test Plan: Fingers crossed! -- I don't have a mac

Reviewers: jvoll, benkraft, jacqueline

Reviewed By: jacqueline

Subscribers: jacqueline, samuel

Differential Revision: https://phabricator.khanacademy.org/D57298

mac-setup.sh

@@ -300,7 +300,11 @@ install_node() {
 install_go() {
     if ! has_recent_go; then   # has_recent_go is from shared-functions.sh
         info "Installing go\n"
-        brew install [email protected]
+        if brew ls go >/dev/null 2>&1; then
+            brew upgrade [email protected]
+        else
+            brew install [email protected]
+        fi
     else
         success "go already installed"
     fi