Use cli fancy formatting again for build messages

[gadenbuie]

This resolves the issue previously seen in #19 where output_file wasn't found and that was caused by a confused cli_process_start() when called within a function but attached to a different environment (where output_file didn't exist).

This fixes that particular issue by building the message first using cli::format_inline() before calling cli_process_start().

While in there I also updated the printing method. I reversed the phrasing to Building input into output since I think this reads in chains better (the output of one line becomes the input of the next).

I also updated the messages to provide a little more context about where the files are being written by printing the paths relative to the working directory.

[jhelvy]

Really nice job with this, and I like the change to "into" in the descriptions. It's much more clear about what is being built and where it is going. This will also be much nicer once the changes in #30 are implemented as you'll be able to see where something is being built even if the user didn't specify the output directory in output_file.