Running the CLI inside of a Github action triggers an stdin warning

[janpieterz]

See the run here.

Run npx prettier --write . Error: UndefinedParserError: No parser could be inferred for file "stdin". Error: Process completed with exit code 1.

Looks like something is falsy around this detection of TTY on GitHub runners (we've ran into other odd issues with that but I am struggling to find a quick fix).