Docker build failing again due to codecov

[DMRobertson]

Related to https://about.codecov.io/blog/message-regarding-the-pypi-package/

E.g https://github.com/matrix-org/sytest/actions/runs/4684534736/jobs/8301237552#step:6:1290

#19 [linux/amd64 10/10] RUN /venv/bin/pip install -q --no-cache-dir         coverage codecov tap.py coverage_enable_subprocess
#19 1.087 ERROR: Could not find a version that satisfies the requirement codecov (from versions: none)
#19 1.088 ERROR: No matching distribution found for codecov
#19 ERROR: process "/bin/bash -o pipefail -c /venv/bin/pip install -q --no-cache-dir         coverage codecov tap.py coverage_enable_subprocess" did not complete successfully: exit code: 1

#18 [linux/arm64  5/10] RUN python3 -m pip install -q --upgrade pip 
#18 CANCELED
------
 > [linux/amd64 10/10] RUN /venv/bin/pip install -q --no-cache-dir         coverage codecov tap.py coverage_enable_subprocess:
#19 1.087 ERROR: Could not find a version that satisfies the requirement codecov (from versions: none)
#19 1.088 ERROR: No matching distribution found for codecov
------
docker/synapse.Dockerfile:74
--------------------
  73 |     # Pre-install test dependencies installed by `scripts/synapse_sytest.sh`.
  74 | >>> RUN /venv/bin/pip install -q --no-cache-dir \
  75 | >>>         coverage codecov tap.py coverage_enable_subprocess
  76 |     
--------------------
ERROR: failed to solve: process "/bin/bash -o pipefail -c /venv/bin/pip install -q --no-cache-dir         coverage codecov tap.py coverage_enable_subprocess" did not complete successfully: exit code: 1
Error: buildx failed with: ERROR: failed to solve: process "/bin/bash -o pipefail -c /venv/bin/pip install -q --no-cache-dir         coverage codecov tap.py coverage_enable_subprocess" did not complete successfully: exit code: 1