windows-xbuild CI uses ubuntu-18.04 for its gcc 7.3.0 (#784)

GitHub's ubuntu-18.04 environment installs gcc 7.3.0.
The latest ubuntu-20.04 environment installs gcc 9.3.0.
Version 9.3.0 is incompatible with building libpng 1.6.37,
as of 2021-03-04.

Fixes #783

.github/workflows/testing.yml

@@ -133,7 +133,7 @@ jobs:
     strategy:
       matrix:
         bits: [32, 64]
-        os: [ubuntu-latest]
+        os: [ubuntu-18.04]
         include:
           - bits: 32
             arch: i686