ci/appsec: smaller matrix and faster jobs

[Julio-Guerra]

What does this PR do?

1. Fix the Go modules caching that wasn't working at all
2. Reuse a runner as much as possible. For instance, no longer ask a runner for the sake of recompiling a test differently.
3. Remove a lot of matrix cases that will be now covered by go-libddwaf's CI
4. Reduce the amount of macos jobs to its minimum, wich is oldest and latest.

Motivation

• Faster CI
• Less flaky CI

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!

[pr-commenter]

Benchmarks

Benchmark execution time: 2024-02-26 16:40:25

Comparing candidate commit c53a196 in PR branch julio.guerra/ci-gh-enterprise-runner with baseline commit 6a6746f in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 40 metrics, 1 unstable metrics.

[darccio]

@Julio-Guerra If I'm looking at the right place and reading it right, some tests are incredibly improved :)

[RomainMuller]

Some relatively minor comments only