Use once_cell instead of lazy_static

once_cell can do what lazy_static does and more, so replace lazy_static with
once_cell.

See https://docs.rs/once_cell/1.8.0/once_cell/#general-purpose-lazy-evaluation

Cargo.lock

@@ -101,7 +101,6 @@ dependencies = [
  "git2",
  "globset",
  "grep-cli",
- "lazy_static",
  "nix",
  "once_cell",
  "path_abs",