Fix get impact area parameter missing issue (sonic-net#20335)

What is the motivation for this PR?
PR sonic-net#20002 refined PR test template, but forgot to pass build branch param to get impact area yaml, which led to impact area not working

How did you do it?
Pass build branch to impact area yaml

How did you verify/test it?
Before:

DIFF_FOLDERS=
After:
DIFF_FOLDERS='.azure-pipelines '

Signed-off-by: Yael Tzur <[email protected]>

Author: xwjiang-ms

.azure-pipelines/pr_test_template.yml

@@ -47,6 +47,8 @@ jobs:
       - ${{ if eq(parameters.CHECKOUT_SONIC_MGMT, true) }}:
           - checkout: ${{ parameters.SONIC_MGMT_NAME }}
       - template: impacted_area_testing/get-impacted-area.yml
+        parameters:
+          BUILD_BRANCH: $(BUILD_BRANCH)
 
   - job: impacted_area_kvmtest
     displayName: " "