make the entrypoint work regardless of install path

from: https://github.com/balena-io-library/armv7hf-debian-qemu/commit/30c52e7daf6f83ded800a2643bf4663fbc062f2d

resin-xbuild.go

@@ -3,7 +3,8 @@ package main
 import (
 	"log"
 	"os"
-	"os/exec"
+	"os/exec"
+	filepath "path/filepath"
 	"syscall"
 )
 
@@ -38,7 +39,9 @@ func runShell() error {
 }
 
 func main() {
-	switch os.Args[0] {
+	command := filepath.Base(os.Args[0])
+	
+	switch command {
 	case "cross-build-start":
 		crossBuildStart()
 	case "cross-build-end":