Actions: Fix Homebrew install command for SDL2

Prepend the file path with `./` to prevent it being interpreted as a Homebrew
Tap. Add `--build-from-source` to make extra sure we don't try to install a
Bottle.

Author: tmewett

.github/workflows/build.yml

@@ -34,7 +34,8 @@ jobs:
 
     - name: "Compile and install dependencies"
       run: |
-        brew install macos/sdl2.rb sdl2_image dylibbundler
+        brew install --build-from-source ./macos/sdl2.rb
+        brew install sdl2_image dylibbundler
 
     - name: "Compile"
       run: |