qemu-build: Set ARCH before using it

Change-Id: Ic3cd4de7f29f690648c0c026c9b9898699497e9f
Signed-off-by: Joel Stanley <joel@jms.id.au>
diff --git a/qemu-build.sh b/qemu-build.sh
index 49c799e..6404576 100755
--- a/qemu-build.sh
+++ b/qemu-build.sh
@@ -46,6 +46,9 @@
 qemu_dir=${qemu_dir:-${WORKSPACE}/qemu}
 WORKSPACE=${WORKSPACE:-${HOME}/${RANDOM}${RANDOM}}
 
+# Determine the architecture
+ARCH=$(uname -m)
+
 # Docker Image Build Variables:
 build_dir=${build_dir:-/tmp/qemu}
 img_name=${img_name:-qemu-build:${ARCH}}
@@ -53,9 +56,6 @@
 # Deployment Variables
 launch=${launch:-}
 
-# Determine the architecture
-ARCH=$(uname -m)
-
 # Timestamp for job
 echo "Build started, $(date)"