ci: Reduce test parallelism

This commit updates the CI workflow to reduce the number of subsets
for running twister tests from 5 to 2 because:

1. There is a lot of overhead in setting up the test environment
   compared to the actual test run time (e.g. it takes 10 minutes
   for set-up and 5 minutes for test run per subset in case of Linux).

2. Windows, the slowest platform, runner has been updated to a more
   powerful 8-core machine.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>

Author: stephanosio

.github/workflows/ci.yml

@@ -1240,10 +1240,10 @@ jobs:
       fail-fast: false
       matrix:
         testenv: ${{ fromJSON(needs.setup.outputs.testenvs) }}
-        subset: [ 1, 2, 3, 4, 5 ]
+        subset: [ 1, 2 ]
 
     env:
-      SUBSET_COUNT: 5
+      SUBSET_COUNT: 2
       PYTHON_DEPS: https://raw.githubusercontent.com/zephyrproject-rtos/zephyr/main/scripts/requirements.txt
       ZEPHYR_TOOLCHAIN_VARIANT: zephyr