shell/build-putenv.ml fails on MSYS2

[kit-ty-kate]

  #=== ERROR while compiling opam-devel.2.5.0~rc1 ===============================#
  # context     2.4.1 | win32/x86_64 | ocaml.4.14.2 | file://D:/a/opam-repository/opam-repository
  # path        D:\opamroot\default\.opam-switch\build\opam-devel.2.5.0~rc1
  # command     C:\msys64\usr\bin\make.exe opam-devel.install
  # exit-code   2
  # env-file    D:\opamroot\log\opam-devel-3144-c601b3.env
  # output-file D:\opamroot\log\opam-devel-3144-c601b3.out
  ### output ###
  # dune build  -p opam opam.install
  # File "src/core/dune", lines 62-66, characters 0-200:
  # 62 | (rule
  # 63 |   (enabled_if (= %{os_type} "Win32"))
  # 64 |   (targets opam-putenv.exe)
  # 65 |   (deps    opamInject.c)
  # 66 |   (action  (run ocaml %{dep:../../shell/build-putenv.ml} %{targets} %{dep:opam-putenv.c} %{dep:cc64})))
  # (cd _build/default/src/core && D:\opamroot\default\bin\ocaml.exe ../../shell/build-putenv.ml opam-putenv.exe opam-putenv.c cc64)
  # Exception: Invalid_argument "index out of bounds".
  # make: *** [Makefile:127: opam-devel.install] Error 1

Seen in ocaml/opam-repository#28928