fix: remove head branch fetch and checkout that fails on dirty worktree

[andrmoel]

Summary

Removes the git fetch origin <head-ref> and git checkout <head-sha> commands from the "Fetch base branch for comparison" step.

Problem: When a consuming workflow runs yarn install (or similar) before this action, yarn.lock gets modified. The subsequent git checkout of the PR head SHA fails because git refuses to overwrite the dirty file:

error: Your local changes to the following files would be overwritten by checkout:
    yarn.lock
Please commit your changes or stash them before you switch branches.

This has been breaking the "Check test coverage" job on every PR in candidate-backend since the Node 24 upgrade (April 2026), because Node 24's yarn now modifies yarn.lock during install on merge commits.

Why the checkout is unnecessary: actions/checkout@v4 on pull_request events checks out a merge commit that already contains the PR's changes on top of the base branch. diff-cover --compare-branch=origin/master therefore produces the correct diff without needing to switch to the PR head commit.

A previous attempt to fix this with git checkout -f (commit 906d4b1) was merged and reverted within 17 minutes.

Review & Testing Checklist for Human

• Merge this PR, then re-run the "Check test coverage" job on an open candidate-backend PR (e.g. #5339) to verify it passes
• Verify the coverage report comment is correctly posted on the PR

Notes

The only remaining git command in the step is git fetch origin <base-ref>, which is needed so diff-cover can compare against origin/master.

Link to Devin session: https://app.devin.ai/sessions/91f6c3e0d63549379d22705dda69e5fd
Requested by: @andrmoel

[devin-ai-integration]

Original prompt from moeller

Since a while the github action "check test coverage on new" is broken in our PRs.
Here is an example: https://github.com/doctariDev/candidate-backend/pull/5339
And here is another example: https://github.com/doctariDev/candidate-backend/actions/runs/26630467849/job/78477725330?pr=5341

Figure out what is causing the problem and explain it to me.

[devin-ai-integration]

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