ci: Use `uv --frozen` instead of `--locked` #2205

Job	Run time
Check for changes	4s
Check Python code 🐍	1m 54s
Check Rust code 🦀	1m 29s
tests (Rust nightly, coverage)	2m 11s
Build benchmarks 🏋️	3m 54s
tests (Python)	2m 0s
tests (Rust 1.75)	2m 16s
tests (Rust beta)	1m 51s
tests (Rust stable, all features)	1m 32s
tests (Rust stable, no features)	1m 36s
rs-semver-checks / Rust semver-checks 🦀	1m 52s
Upload Python coverage 🐍	12s
Check standard extensions	1m 4s
Required checks 🦀+🐍	1s
	21m 56s

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