Install rexml Ruby gem

See latex3/latex3#1103 (comment)

.github/workflows/main.yml

@@ -43,6 +43,7 @@ jobs:
           set -e
           apt-get -qy update
           apt-get -qy install --no-install-recommends poppler-utils ruby
+          gem install rexml
       - name: Compile example document
         run: ${{ matrix.command }}
       - name: Extract text of the example document