Run kernel demos and unit tests for PR changes (#645)

Run kernel demos and unit tests for PR changes (#645)

* Run kernel demos and unit tests for PR changes

Kernel demos check builds multiple demos from FreeRTOS/FreeRTOS and
unit tests check runs unit tests in FreeRTOS/FreeRTOS. Both of these
checks currently use main branch of FreeRTOS-Kernel. This commits
updates these checks to use the changes in the PR.

Signed-off-by: Gaurav Aggarwal <aggarg@amazon.com>
* Do not specify PR SHA explicitly as that is default

Signed-off-by: Gaurav Aggarwal <aggarg@amazon.com>
* Remove explicit PR SHA from kernel checks

Signed-off-by: Gaurav Aggarwal <aggarg@amazon.com>
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Signed-off-by: Gaurav Aggarwal <aggarg@amazon.com>