Initialize the SHELL environment variable

Because docker doesn't carry the SHELL env varialbe, which seems to be
needed during ff builds

Eg: https://salsa.debian.org/gnome-team/mozjs52/commit/78a44df



Signed-off-by: Ritesh Raj Sarraf <ritesh.sarraf@collabora.com>

debian/rules

@@ -20,6 +20,7 @@ PRODUCT := browser
 include debian/upstream.mk
 
 DIST	:= apertis
+export SHELL := /bin/sh
 
 SYSTEM_LIBS = zlib ffi
 ifeq (,$(filter jessie stretch buster apertis,$(DIST)))