Handle builds outside nix

src/main.rs

@@ -337,7 +337,10 @@ fn format_address_table(m: &HashMap<String, (u32, bool)>) -> String {
 }
 
 fn main() {
-    let version_string = format!("fox32asm {} ({})", env!("NIX_SEMVER"), env!("NIX_GIT_SHA_SHORT"));
+    let version_string = match (option_env!("NIX_SEMVER"), option_env!("NIX_GIT_SHA_SHORT")) {
+        (Some(semver), Some(sha)) => format!("fox32asm {semver} ({sha})"),
+        _ => format!("fox32asm (dev)"),
+    };
     println!("{}", version_string);
 
     let args: Vec<String> = env::args().collect();