check integrity of codecov executable

Co-authored-by: Kevin Backhouse <kevinbackhouse@github.com>

.github/workflows/on_PR_linux_special_buils.yml

@@ -44,7 +44,12 @@ jobs:
           ctest --output-on-failure
           pip install gcovr
           gcovr -r ./../ -x --exclude-unreachable-branches --exclude-throw-branches -o coverage.xml .
+          curl https://keybase.io/codecovsecurity/pgp_keys.asc | gpg --import
           curl -Os https://uploader.codecov.io/latest/linux/codecov
+          curl -Os https://uploader.codecov.io/latest/linux/codecov.SHA256SUM
+          curl -Os https://uploader.codecov.io/latest/linux/codecov.SHA256SUM.sig
+          gpg --verify codecov.SHA256SUM.sig codecov.SHA256SUM
+          shasum -a 256 -c codecov.SHA256SUM
           chmod +x codecov
           ./codecov -f build/coverage.xml