CI: Build binaries on older Linux for broader compatiblity

Older Linux uses older glibc. Software linked against older glibc is
generally *forward-compatible* for running on newer systems. But
software linked against *newer* glibc is often not
*backward-compatible* for running on older systems.

So, we should pretty much build the binaries on whatever popular,
still-supported Linux distro ships with oldest glibc.

This is likely to be Debian or a RHEL-compatible distro, but we can
try starting with the oldest one GitHub Actions supports natively,
which I used here (Ubuntu 20.04 instead of Ubuntu 22.04/latest).

Debian or a RHEL-compatible distro such as Alma Linux or Rocky Linux
could be used via Docker, though, relatively easily.

.github/workflows/build.yml

@@ -22,7 +22,7 @@ jobs:
   build:
     strategy:
       matrix:
-        os: [ ubuntu-latest, windows-latest, macos-latest ]
+        os: [ ubuntu-20.04, windows-latest, macos-latest ]
       fail-fast: false
     runs-on: ${{ matrix.os }}