chore: Run CompCert on the stable branch of libsodium

The master branch might include staging code that is subject to change,
e.g. code including VLAs, which CompCert does not support and thus would
fail on, which will be changed to not use VLAs once merged into the
stable branch.

See jedisct1/libsodium#1188 (comment)

other/docker/compcert/compcert.Dockerfile

@@ -13,7 +13,7 @@ COPY --from=sources /src/ /work/
 
 SHELL ["/bin/bash", "-o", "pipefail", "-c"]
 
-RUN git clone --depth=1 https://github.com/jedisct1/libsodium /work/libsodium
+RUN git clone --depth=1 --branch=stable https://github.com/jedisct1/libsodium /work/libsodium
 COPY other/docker/compcert/Makefile /work/
 RUN make "-j$(nproc)"
 RUN ./send_message_test | grep 'tox clients connected'