fix: add optional value label for environment variables

[ferruhcihan]

📌 Summary

This PR fixes 400 error caused by optional value label for environment variables in build creation.

Ticket: https://jira.linode.com/browse/APL-1011

🔍 Reviewer Notes

Considerations

• I have tested the changes in both light and dark mode.
• I have considered the need for new unit tests.
• I have tested the changes on a cluster.
• I have included relevant documentation updates.
• I have an approved Figma design or have reflected my changes in Figma
• I have verified that the UI/UX is consistent in major browsers (e.g., Chrome, Firefox, Safari, Edge).
• I have tested the changes for responsiveness in different screen resolutions.
• I have tested expected error states and verified that the user is presented with informative error messages.
• I have tested the feature with unusual or extreme inputs (e.g., very long strings, empty states, clicking a button multiple times quickly).