cargo deny: allow MPL-2.0 for "options-ext" via "dirs"

* MPL-2.0 - Mozilla Public License 2.0:
├   - OSI approved
├   - FSF Free/Libre
├   - Copyleft
├ option-ext v0.2.0
└── dirs-sys v0.4.1
    └── dirs v5.0.1
        ├── garden-tools v1.7.1-beta0
        └── shellexpand v3.1.0
            └── garden-tools v1.7.1-beta0 (*)

deny.toml

@@ -3,6 +3,7 @@ allow = [
   # encoding_rs
   "BSD-3-Clause",
   "MIT",
+  "MPL-2.0",
   # clap -> clap_derive -> proc-macro2 -> unicode-ident
   "Unicode-DFS-2016"
 ]