.gitlab/macrobenchmarks.yml: fix CI configuration, substitute master for main on execution policy

[igoragoli]

What does this PR do?

This PR fixes the macrobenchmark CI configuration, enabling macrobenchmarks to be run on commits merged to the main branch, instead of the master branch, as was mistakenly set before.

Related PR: #2285

Motivation

Reviewer's Checklist

• Changed code has unit tests for its functionality at or near 100% coverage.
• 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.

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: 2023-10-31 12:12:45

Comparing candidate commit c4f3a93 in PR branch igoragoli/update-go-macrobenchmark with baseline commit 7f31c76 in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 41 metrics, 0 unstable metrics.