sdl12-compat: CI: Maybe fixed github workspace var

SDL Commits
1 
From 5a0df0d9474112dfb8f2ab468b21e8c366250b7a Mon Sep 17 00:00:00 2001
From: "Ryan C. Gordon" <[EMAIL REDACTED]>
Date: Tue, 1 Jun 2021 18:15:26 -0400
Subject: [PATCH] CI: Maybe fixed github workspace var

I have no idea what I'm doing.  :/
---
 .github/workflows/main.yml | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/.github/workflows/main.yml b/.github/workflows/main.yml
index 5df4c11..77b9223 100644
--- a/.github/workflows/main.yml
+++ b/.github/workflows/main.yml
@@ -27,7 +27,7 @@ jobs:
     - name: Get sdl12-compat sources
       uses: actions/checkout@v2
     - name: Configure CMake
-      run: cmake -DSDL2_INCLUDE_DIR="$GITHUB_WORKSPACE/SDL2/include" -B build ${{ matrix.platform.flags }}
+      run: cmake -DSDL2_INCLUDE_DIR="${{ github.workspace }}/SDL2/include" -B build ${{ matrix.platform.flags }}
     - name: Build
       run: cmake --build build/