examples: lang: Test that each of the mcl examples compiles

We let these rot, so fixup the issues and test them!

examples/lang/env-bad.mcl

@@ -1,7 +1,7 @@
 import "fmt"
 import "sys"
 
-$x = sys.getenv("TEST", "321")
+$x = sys.defaultenv("TEST", "321")
 
 print "print1" {
 	msg => fmt.printf("TEST is: %s", $x),