dune: fix binary corruption during parallel promotion/install

Fixes #6668

When performing multiple promotions in parallel (e.g. for a .bc and an .exe)
then sharing a single global buf means that one of them could get corrupted
if the buffer is overwritten by the other build.

Do not use global variables: allocate a new per-file buffer in 'parse' instead!

`cd test && dune build @unit-tests/artifact_substitution/runtest @blackbox-tests/test-cases/versioncorruption` passes now

Signed-off-by: Edwin Török <edvin.torok@citrix.com>

Author: edwintorok

src/dune_rules/artifact_substitution.ml

@@ -415,8 +415,6 @@ end
 
 let buf_len = max_len
 
-let buf = Bytes.create buf_len
-
 type mode =
   | Test
   | Copy of
@@ -469,6 +467,7 @@ output the replacement        |                                             |
     v} *)
 let parse ~input ~mode =
   let open Fiber.O in
+  let buf = Bytes.create buf_len in
   let rec loop scanner_state ~beginning_of_data ~pos ~end_of_data ~status =
     let scanner_state = Scanner.run scanner_state ~buf ~pos ~end_of_data in
     let placeholder_start =