ci: Re-enable testing on Arch Linux

This reverts commit 17ab0dfcf4.

The underlying issue with the archlinux was fixed, see references in the
above commit for more details.

.github/workflows/tests.yaml

@@ -36,6 +36,9 @@ jobs:
           - name: "OpenSUSE 15.0"
             runner: ubuntu-latest
             dockerfile: opensuse1500
+          - name: "Archlinux Base (Rolling)"
+            runner: ubuntu-latest
+            dockerfile: arch
           - name: "Mac OS X 10.13"
             runner: macos-latest
             dockerfile: osx