Fix calculating the branch name if BRANCH_NAME isn't set. (#21115)

BRANCH_NAME is the variable Jenkins used, so add support for the
variable Azure DevOps uses (BUILD_SOURCEBRANCH).

Backport of #21114

---------

Co-authored-by: Rolf Bjarne Kvinge <rolf@xamarin.com>

Make.config

@@ -56,7 +56,11 @@ PACKAGE_HEAD_REV=$(shell git rev-parse HEAD)
 # This is done in Make.versions, not here.
 #
 
-ifeq ($(BRANCH_NAME),)
+ifneq ($(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})
+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)
 else