ci: Add new linters.

- fatcontext - Detects nested contexts in loops.
- mirror - Reports wrong mirror patterns of bytes/strings usage.
- nilnil - Checks that there is no simultaneous return of nil error and
  an invalid value.
- usestdlibvars - detect the possibility to use variables/constants from
  the Go standard library.

.golangci.yml

@@ -14,6 +14,7 @@ linters:
     - errorlint
     - exhaustive
     - exportloopref
+    - fatcontext
     - goconst
     - godot
     - gofmt
@@ -22,8 +23,10 @@ linters:
     - govet
     - ineffassign
     - makezero
+    - mirror
     - misspell
     - nilerr
+    - nilnil
     - nosprintfhostport
     - prealloc
     - predeclared
@@ -33,5 +36,6 @@ linters:
     - tparallel
     - typecheck
     - unconvert
+    - usestdlibvars
     - unparam
     - unused