Fix cbmc installation in CI

The step 'install latest CBMC' in 'run_cbmc_proofs' CI workflow seems to be failing sporadically.
Sometimes the failure disappears when the CI job is re-run but sometimes the failure does not disappear.

The error looks like:

jq: error (at <stdin>:1): Cannot index string with string "assets"
Error: Process completed with exit code 1.

This commit tries to fix the error by changing the source of CBMC package to https://api.github.com/repos/diffblue/cbmc/releases/latest.

.github/workflows/proof_ci.yaml

@@ -60,9 +60,9 @@ jobs:
         if: ${{ env.CBMC_VERSION == 'latest' }}
         shell: bash
         run: |
-          # Search within 5 most recent releases for latest available package
-          CBMC_REL="https://api.github.com/repos/diffblue/cbmc/releases?page=1&per_page=5"
-          CBMC_DEB=$(curl -s $CBMC_REL | jq -r '.[].assets[].browser_download_url' | grep -e 'ubuntu-20.04' | head -n 1)
+          # Install the latest available package
+          CBMC_REL="https://api.github.com/repos/diffblue/cbmc/releases/latest"
+          CBMC_DEB=$(curl -s $CBMC_REL | jq -r '.assets[].browser_download_url' | grep -e 'ubuntu-20.04' | head -n 1)
           CBMC_ARTIFACT_NAME=$(basename $CBMC_DEB)
           curl -o $CBMC_ARTIFACT_NAME -L $CBMC_DEB
           sudo dpkg -i $CBMC_ARTIFACT_NAME