Add support for `DUNE_ROOT` environment variable

[sir4ur0n]

Desired Behavior

Please add support for a generic DUNE_ROOT env var, similar to passing --root in the command line.

Rationale

Sometimes Dune guesses wrong the root.
E.g.:

| a/
    |- dune-workspace
    |- b/
        |- dune-workspace

If I am in directory b/ and run dune build, it uses as root directory a/, and I have to instead use dune build --root . every time to work around that.

There are various reasons why one may not want to use the parent directory as root, like:

• if a/dune (or another dune file in a/) is invalid/broken, running dune build in b/ will fail, while running dune build --root=. will succeed
• evaluating all dune files in a/ might be slow, thus unnecessarily slowing building b/
• it can mess paths, e.g. we have a Makefile in b/ which refers to _build/<whatever>. Running make foo in b/ will use different paths thus fail or have a wrong behavior

About supporting the env var in particular, it is because it is MUCH more convenient than passing a CLI parameter every time. Env vars can be set once and all subsequent commands will use it, and they interact nicely with many tools (e.g. nix-shell). Comparatively, it is rarely convenient or even possible to pass extra CLI parameters via other tools.

Note: my examples are about dune build but any dune command supporting --root should also support DUNE_ROOT env var.

[nojb]

Sounds reasonable. I think the change should be easy, just adding an ~env argument in

dune/bin/common.ml

Lines 200 to 211 in ce9725f

	let+ root =
	Arg.(
	value
	& opt (some dir) None
	& info
	[ "root" ]
	~docs
	~docv:"DIR"
	~doc:
	"Use this directory as workspace root instead of guessing it. Note that \
	this option doesn't change the interpretation of targets given on the \
	command line. It is only intended for scripts.")

PR welcome!