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.

tests/functional/impure-env.sh

@@ -0,0 +1,33 @@
+source common.sh
+
+# Needs the config option 'impure-env' to work
+requireDaemonNewerThan "2.18.0pre20230816"
+
+enableFeatures "configurable-impure-env"
+restartDaemon
+
+varTest() {
+    local var="$1"; shift
+    local value="$1"; shift
+    nix build --no-link -vL --argstr var "$var" --argstr value "$value" --impure "$@" --file impure-env.nix
+    clearStore
+}
+
+clearStore
+startDaemon
+
+varTest env_name value --impure-env env_name=value
+
+echo 'impure-env = set_in_config=config_value' >> "$NIX_CONF_DIR/nix.conf"
+set_in_config=daemon_value restartDaemon
+
+varTest set_in_config config_value
+varTest set_in_config client_value --impure-env set_in_config=client_value
+
+sed -i -e '/^trusted-users =/d' "$NIX_CONF_DIR/nix.conf"
+
+env_name=daemon_value restartDaemon
+
+varTest env_name daemon_value --impure-env env_name=client_value
+
+killDaemon