oauth2_proxy: Handle attributes being derivations

[mkaito]

Motivation for this change

Any attribute being a derivation would cause infinite recursion

Things done

Handle derivations separately from sets when mapping the config to CLI arguments.

• Tested using sandboxing (nix.useSandbox on NixOS, or option build-use-sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.

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[mkaito]

I'm using toString but apparently that's not quite the same as ${attr}. I know this works because I'm already using it, but maybe someone has an opinion on it? I went with toString because it seemed more expressive to me in this case.