`pixi auth login` does not respect `$RATTLER_AUTH_FILE`

[0xbe7a]

Checks

• [X] I have checked that this issue has not already been reported.

• [X] I have confirmed this bug exists on the latest version of pixi, using pixi --version.

Reproducible example

➜ ~ export RATTLER_AUTH_FILE=$(mktemp) ➜ ~ pixi auth login registry.com --username myusername --password mypassword
Authenticating with .registry.com ➜ ~ cat $RATTLER_AUTH_FILE ➜ ~ cat .rattler/credentials.json {".registry.com":{"BasicHTTP":{"username":"myusername","password":"mypassword"}}}}

Issue description

For pixi auth login pixi falls back to using the default rattler auth backends so 1) Keychain 2) ~/.rattler/credentials.json and so on

Expected behavior

If $RATTLER_AUTH_FILE is set, pixi should only use this backend for all subcommands, including login, and disable all other auth backends.