ci: Fix backend workflow to cache venv using exact python version

[thomass-dev]

Cache venv using exact python version to fix failing pipelines https://github.com/probabl-ai/skore/actions/runs/13260129137/job/37014601717?pr=1305.

References:

https://github.com/actions/setup-python/blob/main/docs/advanced-usage.md#check-latest-version
https://github.com/actions/setup-python/blob/main/docs/advanced-usage.md#python-version

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[github-actions]

Coverage Report for backend

File	Stmts	Miss	Cover	Missing
venv/lib/python3.12/site-packages/skore
__init__.py	14	0	100%
__main__.py	8	8	0%	3–19
exceptions.py	4	4	0%	4–23
venv/lib/python3.12/site-packages/skore/cli
__init__.py	5	5	0%	3–8
cli.py	22	22	0%	3–70
color_format.py	49	49	0%	3–116
venv/lib/python3.12/site-packages/skore/persistence
__init__.py	0	0	100%
venv/lib/python3.12/site-packages/skore/persistence/item
__init__.py	56	3	93%	96–99
altair_chart_item.py	19	1	91%	14
item.py	22	1	95%	86
matplotlib_figure_item.py	36	1	95%	19
media_item.py	22	0	100%
numpy_array_item.py	27	1	94%	16
pandas_dataframe_item.py	29	1	94%	14
pandas_series_item.py	29	1	94%	14
pickle_item.py	22	0	100%
pillow_image_item.py	25	1	93%	15
plotly_figure_item.py	20	1	92%	14
polars_dataframe_item.py	27	1	94%	14
polars_series_item.py	22	1	92%	14
primitive_item.py	23	2	91%	13–15
sklearn_base_estimator_item.py	29	1	94%	15
skrub_table_report_item.py	10	1	86%	11
venv/lib/python3.12/site-packages/skore/persistence/repository
__init__.py	2	0	100%
item_repository.py	59	5	91%	15–16, 202–203, 226
venv/lib/python3.12/site-packages/skore/persistence/storage
__init__.py	4	0	100%
abstract_storage.py	22	0	100%
disk_cache_storage.py	33	1	95%	44
in_memory_storage.py	20	0	100%
venv/lib/python3.12/site-packages/skore/persistence/view
__init__.py	2	2	0%	3–5
view.py	5	5	0%	3–20
venv/lib/python3.12/site-packages/skore/project
__init__.py	3	0	100%
_launch.py	150	1	99%	278
_open.py	9	0	100%
project.py	62	1	99%	236
venv/lib/python3.12/site-packages/skore/sklearn
__init__.py	4	0	100%
_base.py	140	14	90%	36, 48, 58, 91, 94, 147–156, 168–>173, 183–184
find_ml_task.py	45	3	91%	72–>88, 85–87
types.py	2	0	100%
venv/lib/python3.12/site-packages/skore/sklearn/_cross_validation
__init__.py	5	0	100%
metrics_accessor.py	140	0	99%	126–>137
report.py	95	0	100%
venv/lib/python3.12/site-packages/skore/sklearn/_estimator
__init__.py	5	0	100%
metrics_accessor.py	255	9	95%	144–153, 178–>187, 186, 226–>228, 252, 279–283, 298, 318
report.py	120	0	99%	213–>219, 221–>223
utils.py	11	11	0%	1–19
venv/lib/python3.12/site-packages/skore/sklearn/_plot
__init__.py	4	0	100%
precision_recall_curve.py	119	1	98%	229–>246, 317
prediction_error.py	95	1	98%	159, 173–>176
roc_curve.py	126	0	100%
utils.py	89	5	93%	23, 47–49, 53
venv/lib/python3.12/site-packages/skore/sklearn/train_test_split
__init__.py	0	0	100%
train_test_split.py	36	2	94%	16–17
venv/lib/python3.12/site-packages/skore/sklearn/train_test_split/warning
__init__.py	8	0	100%
high_class_imbalance_too_few_examples_warning.py	17	3	78%	16–18, 80
high_class_imbalance_warning.py	18	2	88%	16–18
random_state_unset_warning.py	11	1	87%	15
shuffle_true_warning.py	9	0	91%	44–>exit
stratify_is_set_warning.py	11	1	87%	15
time_based_column_warning.py	22	1	89%	17, 69–>exit
train_test_split_warning.py	5	1	80%	21
venv/lib/python3.12/site-packages/skore/ui
__init__.py	0	0	100%
app.py	32	32	0%	3–83
dependencies.py	4	4	0%	3–10
project_routes.py	41	41	0%	3–92
serializers.py	77	77	0%	3–194
server.py	17	17	0%	3–40
venv/lib/python3.12/site-packages/skore/utils
__init__.py	6	0	100%
_accessor.py	7	0	100%
_environment.py	26	0	97%	29–>34
_logger.py	21	4	84%	14–18
_patch.py	11	5	46%	19–35
_progress_bar.py	30	0	100%
_show_versions.py	31	0	100%
TOTAL	2586	355	86%

Tests	Skipped	Failures	Errors	Time
538	3 💤	0 ❌	0 🔥	52.741s ⏱️

[auguste-probabl]

So we're not actually locking the version, just ensuring that we don't go backwards?