.golangci.yml: sort output by default

So running linter several times will yell the same order of reported
issues, which makes it much easier to address them.

Signed-off-by: Mateusz Gozdek <mgozdekof@gmail.com>

.golangci.yml

@@ -4,6 +4,9 @@ run:
     # Skip autogenerated files.
     - ^.*\.(pb|y)\.go$
 
+output:
+  sort-results: true
+
 linters:
   enable:
     - depguard