Change Path.insert_after_build_dir_exn not to hard code _build

Use the build_dir variable rather than the "_build" string and treat the case
Path.build_dir argument differently. Previously this would error:

Path.insert_after_build_dir_exn Path.build_dir "foo"

Now, it will return _build/foo

src/stdune/path.ml

@@ -468,12 +468,16 @@ let insert_after_build_dir_exn =
       ]
   in
   fun a b ->
-    if not (is_local a) || String.contains b '/' then error a b;
-    match String.lsplit2 a ~on:'/' with
-    | Some ("_build", rest) ->
-      Printf.sprintf "_build/%s/%s" b rest
-    | _ ->
+    if not (is_local a) then
       error a b
+    else if a = build_dir then
+      relative build_dir b
+    else
+      match String.lsplit2 a ~on:'/' with
+      | Some (build_dir', rest) when build_dir = build_dir' ->
+        Local.append (relative build_dir b) rest
+      | _ ->
+        error a b
 
 let rm_rf =
   let rec loop dir =

test/unit-tests/path.mlt

@@ -121,9 +121,7 @@ Called from file "toplevel/toploop.ml", line 180, characters 17-56
 
 Path.insert_after_build_dir_exn Path.build_dir "foobar"
 [%%expect{|
-Exception: Stdune__Exn.Code_error <abstr>.
-Raised at file "src/stdune/exn.ml", line 30, characters 37-131
-Called from file "toplevel/toploop.ml", line 180, characters 17-56
+- : Stdune.Path.t = _build/foobar
 |}]
 
 Path.insert_after_build_dir_exn (Path.relative Path.build_dir "qux") "foobar"