Add codecov coverage uploading

[filmaj]

Summary

Describe the goal of this PR. Mention any related Issue numbers.

Requirements (place an x in each [ ])

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

[filmaj]

Hmm why didn't CI run my changes to the workflow file? 🤔
https://github.com/slackapi/slack-github-action/actions/runs/9008262092/job/24749902585

[zimeg]

@filmaj hmm that's super strange. The workflow file changes seem to be noticed but I'd guess the pull_request_target might use the workflow on main with the changes of the PR branch being checked out.

For testing purposes, I'm wondering if changing this to pull_request would fix this? Just to test though. I think you have the magic to use repository secrets on pull request, which motivated the staging environments and custom checkouts in the first place.

[zimeg]

Otherwise, you could full send the change! It looks good to me and I'm thinking running this on main will "just work".

[zimeg]

👏

[filmaj]

Ahhh yeah good eye, I think you're right, as doing these kinds of changes in other repos that use pull_request and not pull_request_target and that seemed to work fine (see this for example)

[zimeg]

Works on main! https://github.com/slackapi/slack-github-action/actions/runs/9008574305/job/24750965740