[ty] Introduce and use `System::env_var` for better test isolation (#… #31

Job	Run time
cargo build (release)	8m 13s
cargo fmt	16s
Determine changes	11s
mkdocs	4m 42s
pre-commit	1s
python package	2m 59s
cargo clippy	4m 4s
cargo test (linux, release)	1s
cargo build (msrv)	1s
cargo test (linux)	1s
cargo fuzz build	6m 29s
benchmarks	0s
cargo shear	26s
cargo test (wasm)	2m 58s
cargo test (windows)	1s
formatter instabilities and black similarity	4m 56s
test scripts	2m 16s
check playground	0s
ecosystem	0s
Fuzz for new ty panics	0s
fuzz parser	0s
test ruff-lsp	0s
	37m 35s

Provide feedback