Consider deprioritizing the environment variable provider

[kate-goldenring]

In spin, application variables can be configured from multiple providers; however, the environment variable provider will always take precedence. This is odd given that it is the most static of the providers. It takes precedence because it is always at the top of the list of providers and the resolve_variable function loops through providers until it gets a value, so providers listed higher up in runtime config get higher precedence.

We could simply append the environment variable provider to the end of the list of providers to reduce it's precedence