gnu: Add exercism and dependencies.

gnu: Add exercism and dependencies.

* gnu/packages/education.scm (exercism): New variable.
* gnu/packages/patches/exercism-disable-self-update.patch: New file.
* gnu/local.mk (dist_patch_DATA): Add it.
* gnu/packages/golang.scm (go-github-com-spf13-afero)
  (go-github-com-spf13-cast)
  (go-github-com-spf13-cobra)
  (go-github-com-spf13-jwalterweatherman)
  (go-github-com-spf13-viper)
  (go-github-com-fsnotify-fsnotify)
  (go-github-com-magiconair-properties)
  (go-github-com-pelletier-go-toml)
  (go-github-com-subosito-gotenv)
  (go-gopkg-in-ini-v1)
  (go-github-com-mitchellh-mapstructure): New variables.