Report BAT_CONFIG_DIR in bat --diagnostic

BAT_CONFIG_DIR overrides the config directory (src/bin/bat/directories.rs) and is documented in the README, but it was missing from the environment list that bat --diagnostic prints, so a bug report never showed where bat was actually reading its config from. Extend diagnostic_sanity_check to cover it.

src/bin/bat/main.rs

@@ -333,6 +333,7 @@ fn invoke_bugreport(app: &App, cache_dir: &Path) -> Result<()> {
         .info(CommandLine::default())
         .info(EnvironmentVariables::list(&[
             "BAT_CACHE_PATH",
+            "BAT_CONFIG_DIR",
             "BAT_CONFIG_PATH",
             "BAT_OPTS",
             "BAT_PAGER",

tests/integration_tests.rs

@@ -1755,6 +1755,7 @@ fn diagnostic_sanity_check() {
         .success()
         .stdout(predicate::str::contains("BAT_PAGER="))
         .stdout(predicate::str::contains("BAT_WIDTH="))
+        .stdout(predicate::str::contains("BAT_CONFIG_DIR="))
         .stderr("");
 }