Fix unnecessary rebuilds due to pyo3 config file modified time change (#2446)

On some of my computers, maturin build always rebuilds pyo3 and friends,
and looking at the cargo fingerprinting, this is caused by the
py03-config file being written regardless of changes. This only writes
the file if it has changed since last rebuild, which seems to prevent
the problem.

---------

Co-authored-by: messense <[email protected]>

Author: TheZoq2

src/compile.rs

@@ -441,12 +441,17 @@ fn cargo_build_command(
                 target_triple, interpreter.major, interpreter.minor
             ));
             fs::create_dir_all(&maturin_target_dir)?;
-            fs::write(&config_file, pyo3_config).with_context(|| {
-                format!(
-                    "Failed to create pyo3 config file at '{}'",
-                    config_file.display()
-                )
-            })?;
+            // We don't want to rewrite the file every time as that will make cargo
+            // trigger a rebuild of the project every time
+            let existing_pyo3_config = fs::read_to_string(&config_file).unwrap_or_default();
+            if pyo3_config != existing_pyo3_config {
+                fs::write(&config_file, pyo3_config).with_context(|| {
+                    format!(
+                        "Failed to create pyo3 config file at '{}'",
+                        config_file.display()
+                    )
+                })?;
+            }
             let abs_config_file = config_file.normalize()?.into_path_buf();
             build_command.env("PYO3_CONFIG_FILE", abs_config_file);
         }