[macOS] Stick to go 1.15 (#2795)

images/macos/provision/core/commonutils.sh

@@ -19,6 +19,9 @@ if is_Less_Catalina; then
     echo "export USE_BAZEL_VERSION=${USE_BAZEL_VERSION}" >> "${HOME}/.bashrc"
 fi
 
+# Create a symlink for Go 1.15 to preserve backward compatibility
+ln -sf $(brew --prefix go@1.15)/bin/go /usr/local/bin/go
+
 # Invoke bazel to download bazel version via bazelisk
 bazel