Opam setenv for path lists is broken on Windows/Cygwin

[MisterDA]

Opam doesn't handle well setenv in Windows / Cygwin, especially with file lists.
To demonstrate, I'm using the opam images for Docker on Windows. The image ships OCaml for Windows from fdopen, and opam 2.0.8.

Dockerfile

FROM  ocaml/opam:windows-mingw-ocaml-4.13
COPY bugs.cmd .
RUN .\bugs.cmd

bugs.cmd

ocaml-env exec -- echo %PKG_CONFIG_PATH%
@rem nothing, ok

ocaml-env exec -- printenv PKG_CONFIG_PATH
@rem nothing, ok

ocaml-env exec -- opam depext -yi conf-pkg-config

ocaml-env exec -- echo %PKG_CONFIG_PATH%
@rem wrong: nothing's exported

ocaml-env exec -- printenv PKG_CONFIG_PATH
@rem good C:/opam/.opam/4.13/lib/pkgconfig

ocaml-env exec -- C:\cygwin64\bin\bash.exe --login -c "printenv PKG_CONFIG_PATH"
@rem C:/opam/.opam/4.13/lib/pkgconfig:C:/opam/.opam/4.13/lib/pkgconfig
@rem wrong: duplicated, uses : as path separator, should use cygwin-style paths

C:\cygwin64\bin\bash.exe --login -c "ocaml-env exec -- printenv PKG_CONFIG_PATH"
@rem C:/opam/.opam/4.13/lib/pkgconfig;C:/opam/.opam/4.13/lib/pkgconfig:
@rem wrong: duplicated, ends with a :, should use cygwin-style paths

For reference, the ENTRYPOINT of the image is ["ocaml-env" "exec" "--64" "--"] and its CMD is cmd.exe.

I have to work around this issue with lines such as

ocaml-env exec -- opam source mirage-crypto.0.10.4
cd mirage-crypto.0.10.4
ocaml-env exec -- opam install -y --deps-only --with-test ./mirage-crypto.opam
ocaml-env exec -- env PKG_CONFIG_PATH=/cygdrive/c/opam/.opam/4.13/lib/pkgconfig dune build -p mirage-crypto @install
ocaml-env exec -- env PKG_CONFIG_PATH=/cygdrive/c/opam/.opam/4.13/lib/pkgconfig opam install -y ./mirage-crypto.opam

# the following would be simpler, but doesn't work (maybe it's another unrelated problem)
ocaml-env exec -- env PKG_CONFIG_PATH=/cygdrive/c/opam/.opam/4.13/lib/pkgconfig opam install -y mirage-crypto

The culprit line in conf-pkg-config is here:

setenv: [PKG_CONFIG_PATH += "%{lib}%/pkgconfig"]