contrib: adjust heptapod CI flavor sizes

Our CI currently runs on a number of different runners, some of which are run
by Clever Cloud.

By setting this environement variable, we're asking for a smaller size runner
for our smaller jobs, which puts less load on the Clever Cloud infrastructure,
while not affecting our overall pipeline times.

contrib/heptapod-ci.yml

@@ -34,6 +34,7 b' checks:'
     variables:
         RUNTEST_ARGS: "--time --test-list /tmp/check-tests.txt"
         PYTHON: python3
+        CI_CLEVER_CLOUD_FLAVOR: S
 
 rust-cargo-test:
     <<: *all
@@ -43,6 +44,7 b' rust-cargo-test:'
         - make rust-tests
     variables:
         PYTHON: python3
+        CI_CLEVER_CLOUD_FLAVOR: S
 
 test-c:
     <<: *runtests