fix: Pass service account private key to integration tests

[Jim-Encord]

https://github.com/encord-team/encord-client-python/actions/runs/11976058761/job/33390853140 was watching this. Please see the warning in the first few lines: Not sure if necessary. How do we type check YAML files 🤔

Download action repository 'actions/upload-artifact@v4' (SHA:b4b15b8c7c6ac21ea08fcf65892d2ee8f75cf882)
Warning: Unexpected input(s) 'private-key-service-account', valid inputs are ['python-version', 'test-report-file', 'private-key', 'private-key-non-org', 'poetry-version', 'integration-tests-location', 'test-dir', 'environment', 'sdk-repository-url']
Run ./.github/actions/run-integration-tests
Prepare all required actions

[github-actions[bot]]

Unit test report (Pydantic 1.x)

180 tests   180 ✅  4s ⏱️   1 suites    0 💤   1 files      0 ❌

Results for commit 9ff9e4a1.

:recycle: This comment has been updated with latest results.

[github-actions[bot]]

Unit test report ((Pydantic 2.x)

180 tests   180 ✅  4s ⏱️   1 suites    0 💤   1 files      0 ❌

Results for commit 9ff9e4a1.

:recycle: This comment has been updated with latest results.