Retry unmount on macOS

[mtelvers]

This may look like a lot of changes, but really, it's just a single line. The new behaviour will now retry the unmount on failure with the addition of the -f flag.

Os.sudo [ "zfs"; "unmount"; "obuilder/result" ]

to

Macos.sudo_fallback [ "zfs"; "unmount"; "obuilder/result" ] [ "zfs"; "unmount"; "-f"; "obuilder/result" ] ~uid:t.uid

Addresses ocurrent/ocaml-ci#975

[shonfeder]

LGTM! Nice clean fix and diagnosis.

[MisterDA]

For me sudo_fallback sounds like the function will try something once, then if it fails try something else in sudo mode. What about adding a ?fallback argument to the sudo function?

[mtelvers]

Yes, I ditched this in favour of #193.