env-vars ignored under exec

Expected Behavior

manual says env-vars apply under exec/build but don't appear to.

Actual Behavior

env-vars are not set.

Reproduction

dune file:

(env
 (_
  (env-vars (XYZ 10))
))

(executable
 (name envar)
)

src file:

let _=
 try
  let v= Sys.getenv "XYZ" in
 print_endline@@"ENV: "^v
 with _->
        print_endline "not found"

running [dune exec ./envar.exe] gets 'not found'.

• PR with a reproducing test:

1. as per the example given

Specifications

• Version of dune (output of dune --version): 2.5.1
• Version of ocaml (output of ocamlc --version) 4.07.1
• Operating system (distribution and version): ubuntu x86-64

Start with the provided dune file and OCaml source, then run the reproduction with dune exec ./envar.exe on the stated Dune and OCaml versions. Check the env-vars behavior under exec and add a reproducing regression test following the testing guidance in HACKING.md. Done means XYZ is available to the executable and the test verifies it.