No ability to configure nix access tokens with specified values

[silvestre]

When running actions on self-hosted GitHub Enterprise (GHES) installations, the github.token provided is specific to the private GHES instance. This token cannot be used to access resources on github.com. While the existing configuration option disable-nix-access-token allows users to disable the usage of github.token as a Nix access token, this workaround fails once the unauthenticated rate limit for api.github.com is exceeded.

Currently, there is no mechanism to configure a custom, working Nix access token for such scenarios. This limitation creates challenges for GHES users who need to:

• Authenticate against api.github.com without exceeding rate limits.
• Configure Nix access tokens for other APIs, such as private hosts, when using Nix packages from private repositories.