ci: Fix flakiness of lnprototest on CI

The lnprototest occasionally fails on our CI due to the
timeout duration we set while waiting for cln to start up.
This timeout is insufficient for machines like the ones
used in the GitHub CI environment.

To address this issue, this commit introduces the use of
environment variables to increase the lnprototest timeout,
ensuring sufficient time for cln to initialize.

Changelog-None
Signed-off-by: Vincenzo Palazzo <vincenzopalazzodev@gmail.com>

.github/workflows/prototest.yaml

@@ -34,6 +34,7 @@ jobs:
           -e TEST_CMD="make check-protos" \
           -e TEST_GROUP=1 \
           -e TEST_GROUP_COUNT=1 \
+          -e TIMEOUT=120 \
           cln-ci-ubuntu
       - name: Upload Unit Test Results
         if: always()