Update build.yml

Author: dankmolot

.github/workflows/build.yml

@@ -31,7 +31,7 @@ jobs:
           - os: windows-latest
             arch: x86
             CMAKE_ARCH_FLAG: -A Win32
-          - os: ubuntu-latest
+          - os: ubuntu-20.04
             arch: x86
             CMAKE_ARCH_FLAG: -DCMAKE_C_FLAGS="-m32" -DCMAKE_CXX_FLAGS="-m32"