xz: force use of the prebuilt.

Also comment why we can't yet do the same for bzip2.

Test: treehugger
Change-Id: I34d5bc0076f5790fcb01c185ef6d08824a4f6142

ui/build/paths/config.go

@@ -74,8 +74,10 @@ func GetConfig(name string) PathConfig {
 }
 
 var Configuration = map[string]PathConfig{
-	"bash":     Allowed,
-	"bc":       Allowed,
+	"bash": Allowed,
+	"bc":   Allowed,
+	// We need bzip2 here even though we provide a bzip2 binary because
+	// GNU tar seems to avoid calling ours.
 	"bzip2":    Allowed,
 	"date":     Allowed,
 	"dd":       Allowed,
@@ -104,7 +106,6 @@ var Configuration = map[string]PathConfig{
 	"timeout":  Allowed,
 	"tr":       Allowed,
 	"unzip":    Allowed,
-	"xz":       Allowed,
 	"zip":      Allowed,
 	"zipinfo":  Allowed,