ci: improve shellcheck job
ci: improve shellcheck job

1. Use env directive instead of adding to $GITHUB_ENV.

2. Use bash herefile to feed sha256sum instead of pipe to grep.

3. Fix the hardcoded checksum (it was missing the first character).

Signed-off-by: Kir Kolyshkin <3a017b8ddb3f9cf3e4a59978b004111bdeb97f08@gmail.com>

opencontainers/runc,opencontainers/runc,opencontainers/runc,opencontainers/runc