nix: Add quiet flag with shortName overriding its common variant

This PR adds `q` as a `shortName` for the `quiet` flag for `nix`
command, overriding
https://github.com/NixOS/nix/blob/26c3fc11eada3fa7df0284190095868a947fefe2/src/libmain/common-args.cc#L23-L28.
Since this flag can be repeated, it simplifies usage by allowing `nix
-qq ...` instead of `nix --quiet --quiet ...`.

src/nix/main.cc

@@ -117,6 +117,14 @@ struct NixArgs : virtual MultiCommand, virtual MixCommonArgs, virtual RootArgs
             .handler = {[this]() { this->helpRequested = true; }},
         });
 
+        addFlag({
+            .longName = "quiet",
+            .shortName = 'q',
+            .description = "Decrease the logging verbosity level.",
+            .category = loggingCategory,
+            .handler = {[]() { verbosity = verbosity > lvlError ? (Verbosity) (verbosity - 1) : lvlError; }},
+        });
+
         addFlag({
             .longName = "print-build-logs",
             .shortName = 'L',