Avoid coverage upload for merge up pull requests

When there are no conflicts between branches, we create pull requests
where the head branch is a branch on the origin repository. That branch
points to a commit that should already have coverage information
provided by the build that happens after merging a regular pull request.

The thing is, coverage information provided by builds that happen before
merging a pull request are associated with the commit of the head
repository. This means that when merging up 1.2 into 1.3, the build
produces coverage information that is the result of a merge between 1.2
and 1.3, and associates it with 1.2, although it is run on with a
codebase that is much closer to 1.3 (and is in fact supposed to become
1.3 after the merge).

This means that when we create a merge up PR from 1.2 to anything else,
the coverage information is going to be wrong until a PR targeting 1.2
gets merged.

I do not think we need coverage about conflictless merge up PRs more
than we need accurate numbers, so I propose we disable the upload for
those instead of, say, trying to associate them with the temporary merge
commit.

.github/workflows/continuous-integration.yml

@@ -363,6 +363,8 @@ jobs:
   upload_coverage:
     name: "Upload coverage to Codecov"
     runs-on: "ubuntu-22.04"
+    # Only run on PRs from forks
+    if: "github.event.pull_request.head.repo.full_name != github.repository"
     needs:
       - "phpunit-smoke-check"
       - "phpunit-postgres"