fix: make binary release compatible with Ubuntu 18.04 in CI

Switching from Travis CI to Github Actions made us switch from Ubuntu 16.04 to
20.04. This causes errors for users running older versions of Ubuntu. I don't
think we can support Ubuntu 16.04 anymore, as it has gone out of support, but
we can at least attempt to support 18.04.

For reference, the reported error seems to be:

    [17871] Error loading Python lib '/tmp/_MEIa1GHWz/libpython3.6m.so.1.0':
    dlopen: /lib/x86_64-linux-gnu/libm.so.6: version GLIBC_2.29’ not found
    (required by /tmp/_MEIa1GHWz/libpython3.6m.so.1.0)`

See: https://discuss.overhang.io/t/error-upgrading-from-v11-2-9-to-v11-2-10/1557

.github/workflows/release.yml

@@ -11,7 +11,7 @@ jobs:
     strategy:
       matrix:
         include:
-          - os: ubuntu-latest
+          - os: ubuntu-18.04
             locale: C.UTF-8
           - os: macos-10.15
             locale: en_US.UTF-8