feat: redirect pg_isready output to /dev/null

Fixed bug where restore commands were called inside pg_isready loop.
Made CommandBuilder::build public. Redirected stdout for pg_isready
calls to /dev/null via Stdio::null().

src/command_builder.rs

@@ -63,7 +63,7 @@ impl CommandBuilder {
         self
     }
 
-    fn build(self) -> Result<Command> {
+    pub fn build(self) -> Result<Command> {
         debug_println!("-----\n{self}\n-----");
 
         let (first, rest) = self.args.split_first().context("empty args")?;