Allow CLI to pass environment variables to FOD builder (#8830)

Add a new experimental `impure-env` setting that is a key-value list of
environment variables to inject into FOD derivations that specify the
corresponding `impureEnvVars`.

This allows clients to make use of this feature (without having to change the
environment of the daemon itself) and might eventually deprecate the current
behaviour (pick whatever is in the environment of the daemon) as it's more
principled and might prevent information leakage.

doc/manual/src/language/advanced-attributes.md

@@ -112,6 +112,13 @@ Derivations can declare some infrequently used optional attributes.
     > environmental variables come from the environment of the
     > `nix-build`.
 
+    If the [`configurable-impure-env` experimental
+    feature](@docroot@/contributing/experimental-features.md#xp-feature-configurable-impure-env)
+    is enabled, these environment variables can also be controlled
+    through the
+    [`impure-env`](@docroot@/command-ref/conf-file.md#conf-impure-env)
+    configuration setting.
+
   - [`outputHash`]{#adv-attr-outputHash}; [`outputHashAlgo`]{#adv-attr-outputHashAlgo}; [`outputHashMode`]{#adv-attr-outputHashMode}\
     These attributes declare that the derivation is a so-called
     *fixed-output derivation*, which means that a cryptographic hash of