Update linux_host_engine.json ci/host_release description (#164402)

Describe what is actually being produced by this target.

This target was just copy/pasted from the debug target (which does
produce more things), but really what this is doing is
compiling/uploading flutter_patched_sdk_product.zip which is used by
*all* release mode targets.

engine/src/flutter/ci/builders/linux_host_engine.json

@@ -98,7 +98,7 @@
                 "--no-goma"
             ],
             "name": "ci/host_release",
-            "description": "Produces release mode Linux host-side tooling.",
+            "description": "Produces flutter_patched_sdk_product.zip shared by all Flutter release builds.",
             "ninja": {
                 "config": "ci/host_release",
                 "targets": [