os: deprecate os.exec (returning ?os.Result), in favour of os.execute, which returns os.Result (#8974)

cmd/tools/vself.v

@@ -32,7 +32,7 @@ fn main() {
 }
 
 fn compile(vroot string, cmd string) {
-	result := os.exec(cmd) or { panic(err) }
+	result := os.execute_or_panic(cmd)
 	if result.exit_code != 0 {
 		eprintln('cannot compile to `$vroot`: \n$result.output')
 		exit(1)