Disable always-authenticate when running under Dependabot (#16773)

Dependabot appears to run a proxy which intercepts all requests and adds
credentials — credentials are _not_ provided via the CLI or environment
variables and there's no way for a user to do so. This means that when
`authenticate = "always"` is used (or when the index URL is on a pyx
domain), uv will fail even though Dependabot may intercept the request
and add credentials.

See
https://github.com/dependabot/dependabot-core/#private-registry-credential-management

docs/reference/environment.md

@@ -791,6 +791,11 @@ Used to determine the name of the active Conda environment.
 
 Used to detect the path of an active Conda environment.
 
+### `DEPENDABOT`
+<small class="added-in">added in `next release`</small>
+
+Used to determine if we're running in Dependabot.
+
 ### `FISH_VERSION`
 <small class="added-in">added in `0.1.28`</small>