Fix `BigInt` formatting negative check #2426

Job	Run time
build_nightly	1m 38s
build_examples	3m 8s
Verify that web-sys is compiled correctly	45s
build_benchmarks	54s
Check	1m 5s
Clippy	2m 55s
Clippy (project)	2m 34s
Clippy (web-sys)	2m 37s
dist_linux_aarch64_gnu	3m 1s
dist_linux_x86_64_musl	3m 9s
dist_macos_aarch64	1m 22s
dist_macos_x86_64	1m 27s
dist_windows	2m 50s
doc_api	1m 14s
doc_book	7s
Check MSRV for CLI tools	34s
Check MSRV for libraries	2m 37s
Rustfmt	6s
Taplo	5s
Build and test the deno example	50s
Run native tests	2m 15s
Run wasm-bindgen crate tests with multithreading	1m 41s
Run UI compile-fail tests	42s
Run wasm-bindgen crate tests (wasm-bindgen)	2m 1s
Run wasm-bindgen crate tests with externref feature	1m 49s
Run wasm-bindgen wasm test	1m 13s
Run wasm-bindgen crate tests with multi-value feature	1m 18s
Run web-sys tests (with geckodriver)	25s
Run wasm-bindgen crate tests (wasm-bindgen (serde-serialize))	1m 28s
Run web-sys (all features) tests (with geckodriver)	2m 51s
Run wasm-bindgen crate tests (wasm-bindgen (enable-interning))	1m 21s
Run web-sys (unstable, all features) tests (with geckodriver)	3m 10s
Run js-sys tests (with geckodriver)	1m 20s
Run js-sys (unstable) tests (with geckodriver)	1m 30s
Run wasm-bindgen-webidl tests (with geckodriver)	28s
Run webidl-tests tests (with geckodriver)	1m 22s
Run webidl-tests (unstable) tests (with geckodriver)	1m 23s
Run typescript-tests tests (with geckodriver)	1m 24s
test_nightly	49s
test_examples	1m 50s
deploy	31s
	1h 3m 49s

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