ci: Remove macOS from test-docs

macOS fails due to missing gvpack, but trying to install graphviz
triggers a Python update which breaks the macOS runner.

Author: KrystalDelusion

.github/workflows/test-build.yml

@@ -125,7 +125,7 @@ jobs:
       CC: clang
     strategy:
       matrix:
-        os: [ubuntu-20.04, macos-13]
+        os: [ubuntu-20.04]
       fail-fast: false
     steps:
       - name: Checkout Yosys