crossbuild.bash: use shell function instead of variable

This will allow easy expansion of build steps.

crossbuild.bash

@@ -1,29 +1,31 @@
-#!/bin/bash -eu
+#!/bin/bash
 #
 # Build on all supported architectures & operating systems
 
+function build {
+	# Discard resulting binary by writing to /dev/null
+	go build -tags without_openssl -o /dev/null
+}
+
+set -eux
+
 cd "$(dirname "$0")"
 
 export GO111MODULE=on
-# Discard resulting binary by writing to /dev/null
-B="go build -tags without_openssl -o /dev/null"
-
-set -x
-
 export CGO_ENABLED=0
 
-GOOS=linux  GOARCH=amd64         $B
+GOOS=linux  GOARCH=amd64         build
 
 # See https://github.com/golang/go/wiki/GoArm
-GOOS=linux  GOARCH=arm   GOARM=7 $B
-GOOS=linux  GOARCH=arm64         $B
+GOOS=linux  GOARCH=arm   GOARM=7 build
+GOOS=linux  GOARCH=arm64         build
 
 # MacOS on Intel
-GOOS=darwin GOARCH=amd64 $B
+GOOS=darwin GOARCH=amd64 build
 
 # MacOS on Apple Silicon M1.
 # Go 1.16 added support for the M1 and added ios/arm64,
 # so we use this to check if we should attempt a build.
 if go tool dist list | grep ios/arm64 ; then
-	GOOS=darwin GOARCH=arm64 $B
+	GOOS=darwin GOARCH=arm64 build
 fi