system: implement Display

ofborg/src/commentparser.rs

@@ -244,5 +244,4 @@ baz",
             parse("@ofborg build foo bar baz.Baz")
         );
     }
-
 }

ofborg/src/outpathdiff.rs

@@ -191,5 +191,4 @@ gnome3.evolution_data_server.aarch64-linux                                 /nix/
         );
         assert_eq!(parse_lines(&mut Cursor::new(TEST_LINES)), expect);
     }
-
 }

ofborg/src/systems.rs

@@ -5,15 +5,17 @@ pub enum System {
     X8664Darwin,
 }
 
-impl System {
-    pub fn to_string(&self) -> String {
+impl std::fmt::Display for System {
+    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
         match self {
-            System::X8664Linux => String::from("x86_64-linux"),
-            System::Aarch64Linux => String::from("aarch64-linux"),
-            System::X8664Darwin => String::from("x86_64-darwin"),
+            System::X8664Linux => write!(f, "x86_64-linux"),
+            System::Aarch64Linux => write!(f, "aarch64-linux"),
+            System::X8664Darwin => write!(f, "x86_64-darwin"),
         }
     }
+}
 
+impl System {
     pub fn as_build_destination(&self) -> (Option<String>, Option<String>) {
         (None, Some(format!("build-inputs-{}", self.to_string())))
     }