vfmt: change all '$expr' to '${expr}' (#16428)

examples/process/command.v

@@ -28,7 +28,7 @@ fn main() {
 	exec("bash -c 'find /tmp/'")
 	out = exec('echo to stdout')
 	out = exec('echo to stderr 1>&2')
-	println("'$out'")
+	println("'${out}'")
 	// THIS DOES NOT WORK, is error, it goes to stderror of the command I run
 	assert out == 'to stderr'
 }