arti: Don't output ANSI escape codes when logging to file.

Note: this only applies when logging to file.

Before:

[2m2024-02-22T18:09:58Z[0m [32m INFO[0m [2marti[0m[2m:[0m Starting Arti 1.1.13 in SOCKS proxy mode on localhost port 9150 ...
[2m2024-02-22T18:09:58Z[0m [34mDEBUG[0m [2marti::process[0m[2m:[0m Increased process file limit to 16384

After:

2024-02-22T18:32:45Z  INFO arti: Starting Arti 1.1.13 in SOCKS proxy mode on localhost port 9150 ...
2024-02-22T18:32:45Z DEBUG arti::process: Increased process file limit to 16384

Closes #1298 (closed)