Work around prove6 bug in CI

Work around Leont/app-prove6#19
by installing the last App::Prove6 release without that bug.

Author: japhb

.github/workflows/test.yml

@@ -31,6 +31,6 @@ jobs:
       - name: Install Dependencies
         run: zef install --/test --test-depends --deps-only .
       - name: Install App::Prove6
-        run: zef install --/test App::Prove6
+        run: zef install --/test App::Prove6:ver<0.0.15>
       - name: Run Tests
         run: prove6 -l -v t