GitHub workflow: document how one can install old Poetry locally

This is quite minor, but it took me a few minutes to figure out
the correct command.

It might be slightly better to print this text inside the build
logs, where people will be looking for certain if the CI fails,
but I didn't immediately find a good way to do so without
complicating the config too much.

.github/workflows/build.yml

@@ -93,6 +93,9 @@ jobs:
           # Test with the version of Poetry in Debian stable. If this starts
           # failing, we should increase this version and document the minimum
           # necessary version of Poetry in contributing.md.
+          #
+          # One way to install old `poetry` is using `pipx`:
+          #    pipx install 'poetry<1.4' --suffix -1.3
           poetry-version: 1.3.2
       - name: Install dependencies
         run: poetry install --no-root