build: make build_source_package(None) default to the current directory

None mapped to Path::new("."), whose parent is the empty string:
the output-directory derivation then always failed with 'cannot
determine output directory', making the documented Option default a
guaranteed-failure trap. Resolve None to the process's absolute
current working directory instead.

src/build/mod.rs

@@ -72,11 +72,19 @@ pub fn build_source_package(
     cwd: Option<&Path>,
     ui: Option<Arc<DebUi>>,
 ) -> Result<(), Box<dyn Error>> {
-    let cwd = cwd.unwrap_or_else(|| Path::new("."));
-    let output = match run_source_build(cwd, &SourceBuildOptions::default(), ui.clone()) {
+    // Default to the process's current working directory, resolved to an
+    // absolute path: the output directory is derived from `cwd.parent()`
+    // downstream, which only yields a real directory for an absolute `cwd`
+    // (the parent of "." is the empty path).
+    let cwd = match cwd {
+        Some(p) => p.to_path_buf(),
+        None => std::env::current_dir()
+            .map_err(|e| format!("cannot determine the current working directory: {e}"))?,
+    };
+    let output = match run_source_build(&cwd, &SourceBuildOptions::default(), ui.clone()) {
         Ok(output) => output,
         Err(e) if e.downcast_ref::<VendorDriftError>().is_some() => {
-            return retry_after_revendor(cwd, ui, e);
+            return retry_after_revendor(&cwd, ui, e);
         }
         Err(e) => {
             if let Some(u) = &ui {