message: [src/doc/book] Update 03-04-environment-variables.md from environment-variables.md
diff: build-script.md book/src/03-05-build-scripts.md
  config.md book/src/03-03-config.md
  crates-io.md book/src/03-06-crates-io.md
  -environment-variables.md
  +environment-variables.md book/src/03-04-environment-variables.md
  external-tools.md
  faq.md
  guide.md
  