Add workflow to manually build the mini agent for testing

[thedavl]

What does this PR do?

Add a manually dispatched workflow that builds the serverless trace mini agent off of a given branch. Defaults to main. Used for testing (build linux + windows binaries on mac).

Motivation

Make testing less painful

Additional Notes

Anything else we should know when reviewing?

How to test the change?

Manually tested

For Reviewers

• If this PR touches code that signs or publishes builds or packages, or handles credentials of any kind, I've requested a review from @DataDog/security-design-and-guidance.
• This PR doesn't touch any of that.

[morrisonlevi]

@DataDog/security-design-and-guidance , can we get a check here?

If this PR touches code that signs or publishes builds or packages, or handles credentials of any kind, I've requested a review from DataDog/security-design-and-guidance.

I'm not 100% sure if it's necessary, but this is the first time we've had a PR maybe need such a review in libdatadog, and I'd rather be safe than sorry.

Edit: feel free to reach out to us on #libdatadog on company slack if there is anything you do not wish to discuss in public.

[Zenithar]

LGTM - Linked to review https://datadoghq.atlassian.net/browse/SDG-910

As the PR concerns GHA, I would recommend to get @DataDog/software-integrity-and-trust approval as part of their CICD security expertise.

[cedricvanrompay-datadog]

Looks good to me! Sorry for the delay

[github-actions]

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[Zenithar]

nit: Using UPX for processes that are started repetitively can add a delay to the FaaS platform warmup. Also UPX packed executable can trigger AV and malware analysis pipelines.

Prefer to ship your binary stripped, and compressed by the archiver, like you do in the next step than artificially reduce the binary size which will run additional code to unpack from memory. To reduce your Rust binary size, I advise you to have a look to this repo - https://github.com/johnthagen/min-sized-rust