Undocumented change related to install dir and workspaces

[alanechang]

In dune version 3.10.0, when you build a package inside a workspace, previous builds are not deleted. In newer versions of dune, this is no longer the case.

Suspect #8350 to be the cause of this change, but have not tested this hypothesis.

Reproduction

Have a poc in this repo.

With dune 3.10.0:

$ dune --version
3.10.0
$ git clone https://github.com/alanechang/dune-ws-install-poc.git
$ cd dune-ws-install-poc/
$ dune build --root=. --workspace=a.ws --only-package=package_a @install
$ ls _build/install/
ws_a/
$ dune build --root=. --workspace=b.ws --only-package=package_b @install
$ ls _build/install/
ws_a/  ws_b/

With dune built from the tip of main:

$ dune.exe --version
3.12.0-586-g581f852
$ git clone https://github.com/alanechang/dune-ws-install-poc.git
$ cd dune-ws-install-poc/
$ dune.exe build --root=. --workspace=a.ws --only-package=package_a @install
$ ls _build/install/
ws_a/
$ dune.exe build --root=. --workspace=b.ws --only-package=package_b @install
$ ls _build/install/
ws_b/