Switch to toybox sed (again).

The pathological case has been fixed, so let's see if it sticks this
time.

Test: treehugger
Change-Id: I043d823b05dd8796764e008c5e16e3563b80cec8
diff --git a/ui/build/paths/config.go b/ui/build/paths/config.go
index e494042..d92649f 100644
--- a/ui/build/paths/config.go
+++ b/ui/build/paths/config.go
@@ -102,7 +102,6 @@
 	"python3":  Allowed,
 	"realpath": Allowed,
 	"rsync":    Allowed,
-	"sed":      Allowed,
 	"sh":       Allowed,
 	"tar":      Allowed,
 	"tr":       Allowed,
@@ -155,6 +154,7 @@
 	"readlink":  LinuxOnlyPrebuilt,
 	"rm":        LinuxOnlyPrebuilt,
 	"rmdir":     LinuxOnlyPrebuilt,
+	"sed":       LinuxOnlyPrebuilt,
 	"seq":       LinuxOnlyPrebuilt,
 	"setsid":    LinuxOnlyPrebuilt,
 	"sha1sum":   LinuxOnlyPrebuilt,