message: allow pinning the rust toolchain version manually
diff: @@ -10,6 +10,7 @@ use rustc_serialize::json::ToJson;
  use rustwide::cmd::{Command, SandboxBuilder};
  use rustwide::logging::{self, LogStorage};
  use rustwide::{Build, Crate, Toolchain, Workspace, WorkspaceBuilder};
  +use std::borrow::Cow;
  use std::path::Path;
  use utils::{copy_doc_dir, parse_rustc_version, CargoMetadata};
  use Metadata;
  @@ -73,8 +74,12 @@ impl RustwideBuilder {
  let workspace = WorkspaceBuilder::new(Path::new(workspace_path), USER_AGENT).init()?;
  workspace.purge_all_build_dirs()?;
  + let toolchain_name = std::env::var("CRATESFYI_TOOLCHAIN")
  + .map(|t| Cow::Owned(t))
  + .unwrap_or_else(|_| Cow::Borrowed("nightly"));
  +
  let toolchain = Toolchain::Dist {
  - name: "nightly".into(),
  + name: toolchain_name,
  };
  Ok(RustwideBuilder {
  