Remove O_NONBLOCK.

[analytically]

O_NONBLOCK on stdin is actually causing issues when reading. When we try to read from stdin (like with cat), it's failing immediately with EAGAIN ("Resource temporarily unavailable") because there's no data immediately available.

For stdin, we typically want blocking behavior because:

• The reader (like cat) expects to be able to block waiting for input
• There's no risk of deadlock on the open operation for stdin since we're opening an already-existing FIFO

Closes #353

[analytically]

@mxpv

[mxpv]

I've fixed the CI in main, could you pls rebase your PR?