[fuchsia] Replace system-temp with isolated-temp.

This change replaces system-temp with isolated-temp. As system-temp is
now deprecated.

Bug: fuchsia:25081
Change-Id: I4f87b69caa95df7c9809c3eb3d0f7e40ef4cddad
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/266974
Reviewed-by: John Rosasco <[email protected]>
Auto-Submit: Benjamin Wright <[email protected]>
Commit-Queue: Mike Klein <[email protected]>

Author: benjwright

build/fuchsia/skqp/skqp.cmx

@@ -9,7 +9,7 @@
         ],
         "features": [
             "vulkan",
-            "system-temp"
+            "isolated-temp"
         ],
         "services": [
             "fuchsia.logger.LogSink",