Make use of `restore-keys`

[DMRobertson]

If we don't have a cache-hit for the poetry-managed venv, we'll end up recreating it from scratch. We could save a little bit of time by restoring from a different cache with a similar name.

We could probably make use of restore-keys here:

setup-python-poetry/action.yml

Lines 65 to 70 in 71e0c44

	- name: Restore/cache poetry venv
	id: poetry-venv-cache
	uses: actions/cache@v2
	with:
	path: ${{ steps.poetry-venvs.outputs.dir }}
	key: poetry-venv-${{ runner.os }}-${{ steps.setup-python.outputs.python-version }}-${{ hashFiles('**/poetry.lock') }}-${{ inputs.extras }}

Probably restore-keys: poetry-venv-${{ runner.os }}-${{ steps.setup-python.outputs.python-version }}?

Would need to test it by making a small change to the lockfile and seeing if we modify a cached venv rather than recreating it from scratch. Or we could just make the change and YOLO it.

Either way, it's one for the wishlist.