Add support for .env and custom env files in uv run (#8811)

## Summary

This PR pulls in https://github.com/astral-sh/uv/pull/8263 and
https://github.com/astral-sh/uv/pull/8463, which were originally merged
into the v0.5 tracking branch but can now be committed separately, as
we've made `.env` loading opt-in.

In summary:

- `.env` loading is now opt-in (`--env-file .env`).
- `.env` remains supported on `uv run`, so it's meant for providing
environment variables to the run command, rather than to uv itself.

---------

Co-authored-by: Eduardo González Vaquero <47718648+edugzlez@users.noreply.github.com>

docs/reference/cli.md

@@ -139,6 +139,11 @@ uv run [OPTIONS] [COMMAND]
 
 <p>See <code>--project</code> to only change the project root directory.</p>
 
+</dd><dt><code>--env-file</code> <i>env-file</i></dt><dd><p>Load environment variables from a <code>.env</code> file.</p>
+
+<p>Can be provided multiple times, with subsequent files overriding values defined in previous files.</p>
+
+<p>May also be set with the <code>UV_ENV_FILE</code> environment variable.</p>
 </dd><dt><code>--exclude-newer</code> <i>exclude-newer</i></dt><dd><p>Limit candidate packages to those that were uploaded prior to the given date.</p>
 
 <p>Accepts both RFC 3339 timestamps (e.g., <code>2006-12-02T02:07:43Z</code>) and local dates in the same format (e.g., <code>2006-12-02</code>) in your system&#8217;s configured time zone.</p>
@@ -296,6 +301,9 @@ uv run [OPTIONS] [COMMAND]
 
 </dd><dt><code>--no-editable</code></dt><dd><p>Install any editable dependencies, including the project and any workspace members, as non-editable</p>
 
+</dd><dt><code>--no-env-file</code></dt><dd><p>Avoid reading environment variables from a <code>.env</code> file</p>
+
+<p>May also be set with the <code>UV_NO_ENV_FILE</code> environment variable.</p>
 </dd><dt><code>--no-group</code> <i>no-group</i></dt><dd><p>Exclude dependencies from the specified dependency group.</p>
 
 <p>May be provided multiple times.</p>