chore: update code coverage to use proper python version

[jackwotherspoon]

Code coverage check is failing due to not finding a Python 3.10 distribution.

We are using Python 3.13 in the setup-python section of the coverage job but
then running nox with Python 3.10.

cloud-sql-python-connector/.github/workflows/coverage.yml

Lines 29 to 40 in 27f6de3

	python-version: "3.13"
	- run: pip install nox coverage
	- name: Checkout base branch
	uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
	with:
	ref: ${{ github.base_ref }}
	- name: Calculate base code coverage
	run: |
	nox --sessions unit-3.10

Both setup-python and nox should run using same Python version