Stream build logs to stdout and stderr

[considerate]

This PR modifies the nixBuild function to write the logs to stderr and stdout while the build is in progress.

Closes #24

This is a draft because I haven't actually tested that the logs are printed correctly

• [x] Ensure that the logs are correctly outputted to their respective handles
• [x] Properly decode UTF8 data before converting to String
• [x] Extract streamCommand into separate function
• [x] Support adding a prefix to each line when tee-ing to the default handles.

[considerate]

An observation: Which packages are build are outputted to stderr while building. However, the actual build logs are excluded. Adding the -L flag enables the build logs, I assume that --print-build-logs as well as --log-format raw would do the same.

If that interferes with the --print-out-paths then enabling build logs should of course be avoided.

[jonascarpay]

Nice, this seems to work well. I'm holding off merging until #22 has been taken care of, I suspect there's some cleanup and rebasing to do.

An observation: Which packages are build are outputted to stderr while building. However, the actual build logs are excluded. Adding the -L flag enables the build logs, I assume that --print-build-logs as well as --log-format raw would do the same.

Adding -L seems to work fine, I guess the build logs only go to stderr?

[considerate]

@jonascarpay Thanks for testing this out!

Adding -L seems to work fine, I guess the build logs only go to stderr

I'll go ahead and add --print-build-logs to the nix build command then.

I'd appreciate a message here after #22 is merged so I don't forget to do the rebase.

[jonascarpay]

22 has been merged

[considerate]

@jonascarpay rebased onto master