Fix mistake in branch calculation. (#21129)

Make.config

@@ -59,7 +59,7 @@ PACKAGE_HEAD_REV=$(shell git rev-parse HEAD)
 ifneq ($(BUILD_SOURCEBRANCH),)
 # BUILD_SOURCEBRANCH is set in Azure DevOps, so use that value if it exists
 # it seems to always start with refs/heads/, so strip off that first.
-CURRENT_BRANCH:=$(subst refs/heads,,${BUILD_SOURCEBRANCH})
+CURRENT_BRANCH:=$(subst refs/heads/,,${BUILD_SOURCEBRANCH})
 else ifeq ($(BRANCH_NAME),)
 # BRANCH_NAME is set in Jenkins, so this is for local builds.
 CURRENT_BRANCH:=$(shell git rev-parse --abbrev-ref HEAD)