[2.4 regression] opam install --deps-only does not handle pin dependencies

[kit-ty-kate]

#6375 seems to have broken the following case:

$ cat opam
opam-version: "2.0"
depends: "spdx_licenses"
pin-depends: ["spdx_licenses.dev" "git+https://github.com/kit-ty-kate/spdx_licenses.git"]
$ opam install --deps .
[...]
Nothing to do.

We would expect the pin-depends to be pinned and the new pinned package to be installed.

Encountered in dune's CI by @anmonteiro

[kit-ty-kate]

Fixed in 2.4.1 but moving the milestone to 2.5.0~alpha1 since the PR on master isn't merged yet.