Makefile: Use Bash as Make shell

The default is /bin/sh which can differ among individual workstations.
Explicitly pick Bash to avoid compatibility issues and allow using its
non-POSIX features.

Bug: 193407077
Test: make test
Change-Id: I6832c3e5da774d98952ff82c13c95cb320b7ae8f
diff --git a/Makefile b/Makefile
index f35f896..e640557 100644
--- a/Makefile
+++ b/Makefile
@@ -16,6 +16,7 @@
 
 CPUS ?= $(shell nproc)
 MAKEFLAGS += --jobs=$(CPUS)
+SHELL ?= $(shell which bash)
 
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