Unable to include s-expression files in `env-vars` stanza

Given a dune-workspace file as follows,

(lang dune 2.0)

(env
  (_ (env-vars ("a" "a-value") (:include ./more-vars.sexp))))

... Dune is unable to include values from a sexp file

Expected Behavior

Dune includes values from the s-expression file

Actual Behavior

Dune creates and env value :include with value "./more-vars.sexp"

When extra parens are added around :include, there is a syntax error

File "dune-workspace", line 4, characters 32-59:
4 |   (_ (env-vars ("a" "a-value") ((:include ./more-vars.sexp)))))
                                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^
Error: Atom or quoted string expected

Reproduction

(happy to create a PR with a failing test if this indeed is a bug)

1. Create workspace file

(lang dune 2.0)

(env
  (_ (env-vars ("a" "a-value") ((:include ./more-vars.sexp)))))

2. Create more-vars.sexp with some key value pairs (optional)

("b" "b-value"

3. Run dune exec -- env and look for environment variables a and b (b wont be created)

Specifications

• Version of dune (output of dune --version): 2.8.5
• Version of ocaml (output of ocamlc --version): 4.12.0
• Operating system (distribution and version): Linux

Additional information

Context: I was trying to work around https://github.com/ocaml/dune/issues/108 with a shell script that scans _opam and creates linker flags and hand it down to dune (via a configurator). Users would have to ./load-env.sh && dune build. This was fine and it worked. I was looking for a way to craft the new environment inside configurator but set the environment globally for the entire project, so that users dont have to run the additional ./load-env.sh script before running dune build

Reproduce the behavior with the shown dune-workspace and more-vars.sexp files, then run dune exec -- env and inspect how the env-vars stanza handles :include. Done means the included key-value pairs appear in the command environment without treating :include as a literal variable or raising the shown syntax error.