.gitlab-ci.yml: Use newer image for overning aarch tests

During recent reorganizing of `.gitlab-ci.yml` to work with `tox`, seems
like we missed to update the image used by the overnight aarch tests,
meaning that they currently fail due to `tox` being missing from them.

While these tests will be skipped on MRs usually, here is an example of
what the tests look like if they are actually run with this change:
https://gitlab.com/BuildStream/buildstream/-/jobs/145449561.

Fixes #859.

.gitlab-ci.yml

@@ -61,7 +61,7 @@ tests-ubuntu-18.04:
   <<: *tests
 
 overnight-fedora-28-aarch64:
-  image: buildstream/testsuite-fedora:aarch64-28-06bab030-32a101f6
+  image: buildstream/testsuite-fedora:aarch64-28-5da27168-32c47d1c
   tags:
     - aarch64
   <<: *tests
@@ -70,6 +70,12 @@ overnight-fedora-28-aarch64:
   except: []
   only:
   - schedules
+  before_script:
+    # grpcio needs to be compiled from source on aarch64 so we additionally
+    # need a C++ compiler here.
+    # FIXME: Ideally this would be provided by the base image. This will be
+    # unblocked by https://gitlab.com/BuildStream/buildstream-docker-images/issues/34
+    - dnf install -y gcc-c++
 
 tests-unix:
   # Use fedora here, to a) run a test on fedora and b) ensure that we