fix(ci): do not delete instances from main branch on merge

[gustavovalverde]

Motivation

We were never expecting the ref from a pull_request event would get mixed with the base branch at some point when using:

on:
  workflow_dispatch:
  pull_request:
    branches:
      - main
    types: [closed]

The closed type pulls refs from the base branch instead of the PR branch, causing this workflow to delete instances containing main in the name, instead of the PR name.

Solution

• Change GITHUB_REF_SLUG_URL with GITHUB_HEAD_REF_SLUG_URL to use the head branch from the PR instead of the PR ref, as (at this point) the PR is already merged.

• Delete the clean.yml workflow

Actually closes #3991, the previous PR #4198 didn't fix it, but it provided better diagnostics.

Review

@dconnolly or @teor2345 can review this

Follow Up Work

Later evaluate if automating instance deletion is required, and under which circumstances.