fix: build when git not installed

When the git binary is not available, emit an empty git_hash
so that the build continues instead of aborting due to an
unset environment variable

crates/pacdef_core/build.rs

@@ -1,10 +1,13 @@
 use std::process::Command;
 
 fn main() {
-    let output = Command::new("git")
+    let git_hash = match Command::new("git")
         .args(["rev-parse", "--short", "HEAD"])
         .output()
-        .unwrap();
-    let git_hash = String::from_utf8(output.stdout).unwrap();
+    {
+        Ok(output) => String::from_utf8(output.stdout).expect("git output is utf-8"),
+        _ => String::new(),
+    };
+
     println!("cargo:rustc-env=GIT_HASH={git_hash}");
 }