Linter: Disable shadow linting for ctx and err

It's been a year, and I'm still getting caught out by govet demanding I
don't shadow a var I was deliberately shadowing.
Made worse by an increase in clashes with stylecheck when they both want
opposite things on the same line.

.golangci.yml

@@ -150,6 +150,9 @@ issues:
     - text: "Expect WriteFile permissions to be 0600 or less"
       linters:
         - gosec
+    - text: 'shadow: declaration of "(err|ctx)" shadows declaration at'
+      linters: [ govet ]
+
 
   exclude-dirs:
     - vendor