ci: restore number of workers

Restore number of workers to previous levels since we've used up
the amazon credits.

Signed-off-by: Kumar Gala <[email protected]>

Author: galak

.buildkite/pipeline.yml

@@ -4,7 +4,7 @@ steps:
     env:
       ZEPHYR_TOOLCHAIN_VARIANT: "zephyr"
       ZEPHYR_SDK_INSTALL_DIR: "/opt/sdk/zephyr-sdk-0.12.2"
-    parallelism: 40
+    parallelism: 20
     timeout_in_minutes: 180
     retry:
       manual: true