cmd/makemac: add macOS 10.15 Catalina as an available builder version
cmd/makemac: add macOS 10.15 Catalina as an available builder version

Makemac has been unable to create a macOS 10.15 Catalina builder. The
Catalina image is now available for creation.

Updates golang/go#34748

Change-Id: Ibc6a5f64be4de5d2eb4a89002e29944c77b6627c
Reviewed-on: https://go-review.googlesource.com/c/build/+/207737
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/build,golang/build,golang/build,golang/build