Update zksync-env.sh

[volosianko77]

1. Added more comments to explain the purpose and usage of the script.
2. Used realpath to resolve the absolute path of the repository root directory.
3. Updated command substitution syntax $() instead of backticks ```` for better readability and versatility.
4. Replaced grep and sed commands with more efficient xargs and grep -v commands.
5. Improved error handling and cleanup mechanism.
6. Corrected indentation and spacing for better readability.