[Bug] build-each-commit.py script hard-codes master branch as base

[dlech]

Describe the bug
CI builds targeting the pybricks-v4.x branch are really slowed down because the build-each-commit.py script hard-codes the master branch as the base branch.

https://github.com/pybricks/pybricks-micropython/blob/77384ec20b25115d622fa654505ced9e2e3bc4ed/.github/build-each-commit.py#L45

To reproduce

Run a CI build like: https://github.com/pybricks/pybricks-micropython/actions/runs/13093071041/job/36532011230

Expected behavior
We should only build each commit from the pull request compared to the base branch of the pull request.

Alternative
We can use the master branch for more experimental stuff and start a new branch for the next release.