fixup! amari.xlog: Implement log rotation

Adjust plain writer to append to log file instead of truncating it on
the open. The rotating writers are already ok as they use "a" mode by default.

amari/xlog.py

@@ -494,7 +494,7 @@ def _openwriter(path: str, rotatespec) -> IWriter:
 # _PlainWriter implements writer that emits data to plain file without rotation.
 class _PlainWriter(IWriter):
     def __init__(w, path):
-        w.f = open(path, "w")
+        w.f = open(path, "a")
 
     def writeline(w, line: str):
         w.f.write(line+'\n')