chore: Allow manually triggering integration_tests workflow

[lym953]

What does this PR do?

Allow the workflow integration_tests.yml to be manually triggered by anyone with write access to the repo.

Motivation

@gabedos flagged that the PR #434 created by a customer from a fork didn't trigger the integration test workflow. This is because this workflow only runs on push to the current repo, not a push to a fork or a pull request. We need a way to run integration test on a PR created from a fork.

Testing Guidelines

No test yet. Will merge it and see if we can trigger the workflow from https://github.com/DataDog/datadog-cdk-constructs/actions/workflows/integration_tests.yml

Additional Notes

Types of Changes

• Bug fix
• New feature
• Breaking change
• Misc (docs, refactoring, dependency upgrade, etc.)

Check all that apply

• This PR's description is comprehensive
• This PR contains breaking changes that are documented in the description
• This PR introduces new APIs or parameters that are documented and unlikely to change in the foreseeable future
• This PR impacts documentation, and it has been updated (or a ticket has been logged)
• This PR's changes are covered by the automated tests
• This PR collects user input/sensitive content into Datadog