config: privatize some Cfg fields

Author: djc

src/config.rs

@@ -234,13 +234,13 @@ pub(crate) struct Cfg<'a> {
     pub profile_override: Option<Profile>,
     pub rustup_dir: PathBuf,
     pub settings_file: SettingsFile,
-    pub fallback_settings: Option<FallbackSettings>,
+    fallback_settings: Option<FallbackSettings>,
     pub toolchains_dir: PathBuf,
-    pub update_hash_dir: PathBuf,
+    update_hash_dir: PathBuf,
     pub download_dir: PathBuf,
     pub tmp_cx: temp::Context,
     pub toolchain_override: Option<ResolvableToolchainName>,
-    pub env_override: Option<LocalToolchainName>,
+    env_override: Option<LocalToolchainName>,
     pub dist_root_url: String,
     pub quiet: bool,
     pub current_dir: PathBuf,