Switch to toybox expr(1).

Test: treehugger
Change-Id: Ib015c70841504d74fd0272a8ca6ecb3c617fb759
diff --git a/ui/build/paths/config.go b/ui/build/paths/config.go
index 0f36fba..e5e0821 100644
--- a/ui/build/paths/config.go
+++ b/ui/build/paths/config.go
@@ -83,7 +83,6 @@
 	"dd":        Allowed,
 	"diff":      Allowed,
 	"egrep":     Allowed,
-	"expr":      Allowed,
 	"find":      Allowed,
 	"fuser":     Allowed,
 	"getconf":   Allowed,
@@ -159,6 +158,7 @@
 	"du":       Toybox,
 	"echo":     Toybox,
 	"env":      Toybox,
+	"expr":     Toybox,
 	"head":     Toybox,
 	"id":       Toybox,
 	"mkdir":    Toybox,