[ensure] Don't recompute environment if up to date (#1625)

## Summary

This bug was introduced in
#1584 and has at least 2
effects:

* Perf regression (because we recompute environment all the time)
* Plugins that create flakes may break if they create additional files
in the same directory. (Another PR will address this issue in more
depth)

## How was it tested?

```bash
devbox add mysql80
devbox services up -b
devbox services stop
```

Author: mikeland73

internal/impl/packages.go

@@ -228,8 +228,12 @@ func (d *Devbox) ensurePackagesAreInstalled(ctx context.Context, mode installMod
 	if err != nil {
 		return err
 	}
-	// if mode is ensure and we are up to date, then we can skip the rest
-	if mode == ensure && upToDate {
+
+	if mode == ensure {
+		// if mode is ensure and we are up to date, then we can skip the rest
+		if upToDate {
+			return nil
+		}
 		fmt.Fprintln(d.stderr, "Ensuring packages are installed.")
 	}