Add environment variable for webhook notification on JC build finish

[OriNachum]

Search before asking

• I have searched the jetson-containers issues and found no similar feature requests.

Description

Add environment variable so when jc finishes it sends an update via webhook

Use case

Start a build and get updated wherever the builder wants (Discord, slack, n8n or any other webhook complaint system)

Additional

No response

Are you willing to submit a PR?

• Yes I'd like to help by submitting a PR!

[OriNachum]

@tokk-nv I assess it will help me approve PRs faster, as I trigger a build and wait for the update to get to me.

[OriNachum]

Currently aligning to Discord structure