test: Fix slack payload in integration workflow

[ykim-akamai]

📝 Description

Needed to use job status instead of step outcome
e.g.

"text": "*Build Result:*\n${{ steps.integration_tests.outcome == 'success' && ':large_green_circle: Build Passed' || ':red_circle: Build Failed' }}"

Instead need to use,

"text": "*Build Result:*\n${{ needs.integration_tests.result == 'success' && ':large_green_circle: Build Passed' || ':red_circle: Build Failed' }}"

✔️ How to Test

Tested on forked repo - https://github.com/ykim-1/linodego/actions/runs/11075403223

📷 Preview

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[yec-akamai]

Nice fix!