Support UV_NO_EDITABLE where --no-editable is supported (#15107)

Specifically, support `UV_NO_EDITABLE=1 uv export`. It's now also
supported in `uv add`, though it's default there anyway and the env var
exists only for completeness.

Fixes #15103

docs/reference/cli.md

@@ -1531,7 +1531,7 @@ uv export [OPTIONS]
 </dd><dt id="uv-export--no-dev"><a href="#uv-export--no-dev"><code>--no-dev</code></a></dt><dd><p>Disable the development dependency group.</p>
 <p>This option is an alias of <code>--no-group dev</code>. See <code>--no-default-groups</code> to disable all default groups instead.</p>
 </dd><dt id="uv-export--no-editable"><a href="#uv-export--no-editable"><code>--no-editable</code></a></dt><dd><p>Export any editable dependencies, including the project and any workspace members, as non-editable</p>
-</dd><dt id="uv-export--no-emit-package"><a href="#uv-export--no-emit-package"><code>--no-emit-package</code></a>, <code>--no-install-package</code> <i>no-emit-package</i></dt><dd><p>Do not emit the given package(s).</p>
+<p>May also be set with the <code>UV_NO_EDITABLE</code> environment variable.</p></dd><dt id="uv-export--no-emit-package"><a href="#uv-export--no-emit-package"><code>--no-emit-package</code></a>, <code>--no-install-package</code> <i>no-emit-package</i></dt><dd><p>Do not emit the given package(s).</p>
 <p>By default, all of the project's dependencies are included in the exported requirements file. The <code>--no-emit-package</code> option allows exclusion of specific packages.</p>
 </dd><dt id="uv-export--no-emit-project"><a href="#uv-export--no-emit-project"><code>--no-emit-project</code></a>, <code>--no-install-project</code></dt><dd><p>Do not emit the current project.</p>
 <p>By default, the current project is included in the exported requirements file with all of its dependencies. The <code>--no-emit-project</code> option allows the project to be excluded, but all of its dependencies to remain included.</p>