Allow compile time default paths for cheats and config

This CL makes it easier to install navi on multi-tenant Linux systems, by allowing to provide pre-install cheatsheets in shared folders (and potentially pre-installed config in shared folders).

This does not break or change the existing behaviour. The shared directory / config file is only used if
- There is not already a directory / config file in the user XDG location
- Navi is compiled with NAVI_CONFIG_FILE and/or NAVI_CHEATS_DIRECTORY env var set
- The paths defined in above env vars exist

.github/workflows/ci.yml

@@ -43,6 +43,20 @@ jobs:
           toolchain: stable
           override: true
 
+      - name: Prep environment to test compiled-in paths
+        run: |
+          mkdir /tmp/cheats-dir
+          touch /tmp/config-file
+
+      - name: Run cargo test (with compiled-in paths)
+        uses: actions-rs/cargo@v1
+        continue-on-error: false
+        env:
+          NAVI_CHEATS_DIRECTORY: /tmp/cheats-dir
+          NAVI_CONFIG_FILE: /tmp/config-file
+        with:
+          command: test
+
       - name: Run cargo test
         uses: actions-rs/cargo@v1
         continue-on-error: false