pw_build: Fix output_logs_ unused warning

This variable is only used when being built with the right toolchain.
In order to set this variable in fuchsia and not have GN complain it's
unused, it must be set as not_needed in other situations.

Bug: b/295524695
Change-Id: I9c1b235a49acbe7d2c4f41274146e5aa079cdfb6
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/166991
Reviewed-by: Anthony DiGirolamo <tonymd@google.com>
Commit-Queue: Auto-Submit <auto-submit@pigweed-service-accounts.iam.gserviceaccount.com>

pw_build/python_venv.gni

@@ -79,6 +79,14 @@ template("pw_python_venv") {
                  "_generated_requirements_file",
                ])
   }
+  _output_logs = true
+  if (defined(invoker.output_logs)) {
+    _output_logs = invoker.output_logs
+  }
+  if (!defined(invoker.output_logs) ||
+      current_toolchain != pw_build_PYTHON_TOOLCHAIN) {
+    not_needed([ "_output_logs" ])
+  }
 
   _source_package_labels = []
   foreach(pkg, _source_packages) {
@@ -197,12 +205,6 @@ template("pw_python_venv") {
 
   if (defined(invoker.source_packages) || defined(invoker.requirements)) {
     if (current_toolchain == pw_build_PYTHON_TOOLCHAIN) {
-      if (defined(invoker.output_logs)) {
-        _output_logs = invoker.output_logs
-      } else {
-        _output_logs = true
-      }
-
       # Compile requirements with hashes
       pw_python_action("${target_name}._compile_requirements") {
         module = "piptools"