Retry pipe EAGAIN on macOS

[0x2b3bfa0]

E... AGAIN, blame on me, from the last bullet point on #703; I fixed an edge case just to break another one.

We should really get rid of pipe-args.js and use something more or less standard like get-stdin as hinted in #67 (comment).

Maintaining yet another cross–platform pipe support library on our own doesn't look like a great idea.

[casperdcl]

ideally merge #733 first to prove it was failing? :)

[casperdcl]

https://github.com/iterative/cml/actions/runs/1261868918 macos still seems to pass without this PR owing to cml publish --version working.

Presumably cml publish --md will fail but can't test without comment spam I presume?

[0x2b3bfa0]

Presumably cml publish --md will fail but can't test without comment spam I presume?

Correctly presumed, both of the times. Testing cml publish implies spamming a repository; either this one (argh!) or an ephemeral one created just for each test (variation of #538)