Make the example notebooks run faster

[RobinL]

This has been very irritating to fix!

For some reason, the test runner hung for like 4 minutes when testing notebooks with an interactive widget component

I've also added a caching layer for the dependencies.

Further gains probably possible, but this test is now faster than the main tests, so it's probably good enough

[github-actions]

Test: test_2_rounds_1k_duckdb

Percentage change: -19.0%

	date	time	stats_mean	stats_min	commit_info_branch	commit_info_id	machine_info_cpu_brand_raw	machine_info_cpu_hz_actual_friendly	commit_hash
849	2022-07-12	18:40:05	1.89098	1.87463	splink3	c334bb9	Intel(R) Xeon(R) Platinum 8370C CPU @ 2.80GHz	2.7934 GHz	c334bb9
1536	2023-03-31	09:45:13	1.54142	1.51795	(detached head)	ac5f34e	Intel(R) Xeon(R) Platinum 8171M CPU @ 2.60GHz	2.0951 GHz	ac5f34e

Test: test_2_rounds_1k_sqlite

Percentage change: -10.9%

	date	time	stats_mean	stats_min	commit_info_branch	commit_info_id	machine_info_cpu_brand_raw	machine_info_cpu_hz_actual_friendly	commit_hash
851	2022-07-12	18:40:05	4.32179	4.25898	splink3	c334bb9	Intel(R) Xeon(R) Platinum 8370C CPU @ 2.80GHz	2.7934 GHz	c334bb9
1538	2023-03-31	09:45:13	3.80219	3.79647	(detached head)	ac5f34e	Intel(R) Xeon(R) Platinum 8171M CPU @ 2.60GHz	2.0951 GHz	ac5f34e

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