Allow to run flatpak update with sudo (#738)

This change adds the option `flatpak.use_sudo` that allows to update
the system-wide installation with sudo. When set to `true` the
system-wide installation will be updated with sudo. If set to `false`
(default) the update will be run as regular user.

This solves the problem where running `flatpak update` on a remote
system fails if run as regular user.

Fixes #737.

src/config.rs

@@ -161,6 +161,13 @@ pub struct Firmware {
     upgrade: Option<bool>,
 }
 
+#[derive(Deserialize, Default, Debug)]
+#[serde(deny_unknown_fields)]
+#[allow(clippy::upper_case_acronyms)]
+pub struct Flatpak {
+    use_sudo: Option<bool>,
+}
+
 #[derive(Deserialize, Default, Debug)]
 #[serde(deny_unknown_fields)]
 pub struct Brew {
@@ -221,6 +228,7 @@ pub struct ConfigFile {
     npm: Option<NPM>,
     firmware: Option<Firmware>,
     vagrant: Option<Vagrant>,
+    flatpak: Option<Flatpak>,
 }
 
 fn config_directory(base_dirs: &BaseDirs) -> PathBuf {
@@ -747,6 +755,15 @@ impl Config {
             .unwrap_or(false)
     }
 
+    #[cfg(target_os = "linux")]
+    pub fn flatpak_use_sudo(&self) -> bool {
+        self.config_file
+            .flatpak
+            .as_ref()
+            .and_then(|flatpak| flatpak.use_sudo)
+            .unwrap_or(false)
+    }
+
     #[cfg(target_os = "linux")]
     str_value!(linux, emerge_sync_flags);