Relax memory constraint on CI native stress test to 140M

`time -v` on my machine reports ~137M of total heap usage, with a
maximum GC heap size of 128M. I think this is enough, as we are not
yet looking at optimizing Gears for memory.

Nevertheless it is good to keep an eye on it.

.github/workflows/ci.yml

@@ -56,4 +56,4 @@ jobs:
     - name: Test
       run: sbt rootNative/test
     - name: Stress Test with Lower Memory
-      run: env GC_MAXIMUM_HEAP_SIZE=128M sbt 'rootNative/testOnly StressTest'
+      run: env GC_MAXIMUM_HEAP_SIZE=140M sbt 'rootNative/testOnly StressTest'