Replace lazycell with once_cell

We started to use lazycell because syntect already used it. But syntect has
changed to use once_cell. So we should also do that to prepare for using the
upcoming version of syntect.

Cargo.lock

@@ -102,8 +102,8 @@ dependencies = [
  "globset",
  "grep-cli",
  "lazy_static",
- "lazycell",
  "nix",
+ "once_cell",
  "path_abs",
  "predicates",
  "semver",