Fix Ubuntu 20.04 workflow tests

Previously, the tests did not run because the CMake version shipped with
Ubuntu 20.04 does not understand the `--test-dir` option and silently
fails.

Change-Id: I335e1d9e3890aa56e66a9dfd0fccd4594a84a08c

.github/workflows/linux.yml

diff --git a/.github/workflows/linux.yml b/.github/workflows/linux.yml
index 445d995..1214fd0 100644
--- a/.github/workflows/linux.yml
+++ b/.github/workflows/linux.yml
@@ -79,8 +79,8 @@
 
       - name: Test
         run: |
-          ctest --test-dir build_${{matrix.build_type}} \
-                --config ${{matrix.build_type}} \
+          cd build_${{matrix.build_type}}/
+          ctest --config ${{matrix.build_type}} \
                 --output-on-failure \
                 -j$(nproc)