[red-knot] more efficient UnionBuilder::add (#13411) #33166

Job	Run time
python package	1m 19s
Determine changes	11s
mkdocs	2m 58s
cargo fmt	16s
pre-commit	54s
cargo test (linux)	4m 14s
cargo build (msrv)	3m 25s
cargo clippy	1m 55s
cargo build (release)	5m 23s
cargo test (wasm)	1m 34s
cargo test (windows)	8m 49s
benchmarks	4m 52s
cargo fuzz	5m 14s
cargo shear	29s
formatter instabilities and black similarity	3m 45s
test scripts	57s
test ruff-lsp	28s
ecosystem	0s
Fuzz the parser	0s
	46m 43s

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