Revert "nix3: always use the same verbosity default (info)"

This reverts commit d0390b5cf2.

Other parts of the codebase will need to be adjusted in response to a
default verbosity change. Let's just push this to after 2.90.

Fixes #362.
Fixes #367.

Change-Id: I04648473579146851bda41d764adc1ef954c355d

src/nix/main.cc

@@ -365,6 +365,11 @@ void mainWrapped(int argc, char * * argv)
 
     setLogFormat("bar");
     settings.verboseBuild = false;
+    if (isatty(STDERR_FILENO)) {
+        verbosity = lvlNotice;
+    } else {
+        verbosity = lvlInfo;
+    }
 
     NixArgs args;