Install Source Code Pro using Homebrew

The font casks all use Git instead of SVN now:
Homebrew/homebrew-cask-fonts#6725

And the Google Font appears to have been updated, which should fix the
issue we were getting in VS Code:
microsoft/vscode#100840

systems/mac/Brewfile

@@ -68,6 +68,8 @@ cask "duet"
 cask "elgato-control-center"
 tap "homebrew/cask-versions"
 cask "firefox-developer-edition"
+tap "homebrew/cask-fonts"
+cask "font-source-code-pro"
 cask "google-drive"
 cask "google-chrome"
 cask "hammerspoon"

systems/mac/README.md

@@ -10,11 +10,6 @@
     * Install Bitwarden
         * The Homebrew version doesn't support "Enable browser integration"
 * Install [Homebrew](https://brew.sh/)
-* Install the Source Code Pro font directly from
-    [GitHub](https://github.com/adobe-fonts/source-code-pro)
-    * The Homebrew version needs SVN and uses an [outdated/broken Google Fonts
-      version](https://github.com/Homebrew/homebrew-cask-fonts/issues/3972)
-    * Add the files to Font Book
 * Go through System Settings
     * Apple Id
         * Set the account picture