Rename `is_terminal` to `rustix_is_terminal` in more places. #4

Job	Run time
Rustfmt	5s
Check (stable)	7s
Test (ubuntu-nightly)	12s
Check on Rust nightly (nightly)	13s
Check (nightly)	17s
Test (windows-nightly)	32s
Test (ubuntu-stable)	11s
Test (windows-stable)	1m 34s
Test (macos-nightly)	38s
Test (macos-stable)	17s
	4m 6s

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