Don't recompile dpe every single time

The build script always writes to `arbitrary_max_handles.rs`, which
means that a recompile is always required.

This then triggers a rebuild of any project (i.e., Caliptra) that uses
this.

This fixes is so that we only write the file if the contents need to be
changed, so the build can be cached.

dpe/build.rs

@@ -1,8 +1,7 @@
 // Licensed under the Apache-2.0 license
 
 use std::env;
-use std::fs::File;
-use std::io::Write;
+use std::path::PathBuf;
 
 fn main() {
     let default_value: usize = 24;
@@ -18,13 +17,13 @@ fn main() {
     println!("cargo:rerun-if-env-changed=ARBITRARY_MAX_HANDLES");
     println!("cargo:rerun-if-changed=build.rs");
 
-    let mut file = File::create(&dest_path).unwrap();
-    write!(
-        file,
-        "pub const MAX_HANDLES: usize = {};",
-        arbitrary_max_handles
-    )
-    .unwrap();
+    let max_handles_str = format!("pub const MAX_HANDLES: usize = {};", arbitrary_max_handles);
 
-    println!("cargo:rerun-if-changed={}", dest_path);
+    let dest_path = PathBuf::from(&dest_path);
+    if dest_path.exists() {
+        if std::fs::read_to_string(&dest_path).unwrap_or_default() != max_handles_str {
+            std::fs::write(&dest_path, max_handles_str).unwrap();
+        }
+    }
+    println!("cargo:rerun-if-changed={}", dest_path.display());
 }