Augment `.dir-locals.el'.

* .dir-locals.el: Add more Scheme settings.
* guix-build.in, tests/base32.scm, tests/build-utils.scm,
  tests/builders.scm, tests/derivations.scm, tests/packages.scm,
  tests/snix.scm, tests/store.scm, tests/union.scm, tests/utils.scm:
  Remove redundant Emacs local variable settings.