feat: add GNOROOT environment variables

[gfanton]

This small PR adds the ability to obtain the root directory from the GNOROOT environment variable. If this variable is not set, the root directory is then guessed using the go list command.
This enable the user to manually set gno root dir globally and not bother setting the root-dir parameter every time he need need it

Contributors' checklist...

• Added new tests, or not needed, or not feasible
• Provided an example (e.g. screenshot) to aid review or the PR is self-explanatory
• Updated the official documentation or not needed
• No breaking changes were made, or a BREAKING CHANGE: xxx message was included in the description
• Added references to related issues and PRs
• Provided any useful hints for running manual tests
• Added new benchmarks to generated graphs, if any. More info here.

[moul]

Thank you for your first contribution!

Your PR is related to #682. I recommend discussing with the @harry-hov, the other contributor to merge your changes efficiently. You can propose a plan for merging PRs, their order, or cherry-picking specific changes.

I'll assign each other as reviewers for better collaboration.

[harry-hov]

Suggested change

	// if GNOROOT is not set, try to guess the root directory using the `go list` command.
	cmd := exec.Command("go", "list", "-m", "-mod=mod", "-f", "{{.Dir}}", "github.com/gnolang/gno")
	out, err := cmd.CombinedOutput()
	if err != nil {
	log.Fatal("can't guess --root-dir, please fill it manually.")
	log.Fatal("can't guess --root-dir, please fill it manually or define the GNOROOT environment variable globally.")
	}
	rootDir := strings.TrimSpace(string(out))
	return rootDir

I really want to get rid of this part.

Either use --rootdir or set GNOROOT. no guessing anymore :)

this will affect many commands/functionalities. Since it's a breaking change. If we all agree on this, it can be done in a separate PR.

[zivkovicmilos]

I suggest opening up a separate PR for this change as it has ramifications for existing user functionality

[harry-hov]

LGTM 💯

[zivkovicmilos]

Thank you for the contribution 🙏

Looks good 🚀

[moul]

Let's make sure GNOROOT points to a structured directory, not the clone directory, just like GOROOT. For now, we can assume the repository is already "compatible" with GNOROOT to avoid migration headaches.

To help developers, we must provide detailed documentation for all environment variables, default values/behaviors. And to keep things clear and manageable, we should limit the number of environment variables.

[moul]

I'll merge the changes now.

Before updating the documentation, let's wait for the new behavior by @harry-hov. It won't expect GNOROOT to have an examples folder anymore. Instead, it'll use a folder with cached packages, which will download missing dependencies and could get automatically filled when installing the gno binary or running it for the first time.