Add eval-system option

`eval-system` option overrides just the value of `builtins.currentSystem`.
This is more useful than overriding `system` since you can build these
derivations on remote builders which can work on the given system.

Co-authored-by: John Ericson <John.Ericson@Obsidian.Systems>
Co-authored-by: Valentin Gagarin <valentin.gagarin@tweag.io>

src/libexpr/eval-settings.cc

@@ -89,6 +89,12 @@ std::string EvalSettings::resolvePseudoUrl(std::string_view url)
         return std::string(url);
 }
 
+const std::string & EvalSettings::getCurrentSystem()
+{
+    const auto & evalSystem = currentSystem.get();
+    return evalSystem != "" ? evalSystem : settings.thisSystem.get();
+}
+
 EvalSettings evalSettings;
 
 static GlobalConfig::Register rEvalSettings(&evalSettings);