.github/workflows: fixes apm:ecosystem label for issues and PRs

[katiehockman]

What does this PR do?

Separates the workflows for PRs and issues to add the apm:ecosystem label. We now have two workflows to add the label:

1. Looks for PRs with files in any contrib/* directory
2. Looks for issues with contrib in the title

Motivation

It's broken today, and adding the labels on every issue, which we don't want. For example: #2516.

This broke when https://github.com/DataDog/dd-trace-go/pull/2496/files# was merged, because it stopped filtering the issues based on the title, and started relying on path instead (which in hindsight, won't work). Separating the workflows allows the job to use the if: contains(github.event.issue.title, 'contrib') constraint for issues, and allows a check on the files with contrib/** in the path for pull requests.

Reviewer's Checklist

• Changed code has unit tests for its functionality at or near 100% coverage.
• System-Tests covering this feature have been added and enabled with the va.b.c-dev version tag.
• There is a benchmark for any new code, or changes to existing code.
• If this interacts with the agent in a new way, a system test has been added.
• Add an appropriate team label so this PR gets put in the right place for the release notes.
• Non-trivial go.mod changes, e.g. adding new modules, are reviewed by @DataDog/dd-trace-go-guild.

For Datadog employees:

• If this PR touches code that handles credentials of any kind, such as Datadog API keys, I've requested a review from @DataDog/security-design-and-guidance.
• This PR doesn't touch any of that.

Unsure? Have a question? Request a review!

[nsrip-dd]

Looks good on a skim, so I'm okay rubber-stamping and we can test after it lands :)

[katiehockman]

Looks like it's working as expected now: #2527 #2526