cmd/initContainer: Use errors.Is() instead of os.IsNotExist()

The os.IsNotExist() function [1] predates the introduction of the
errors.Is() function [2] in Go 1.13 [3].  From Go >= 1.16, the
documentation explicitly recommends the use of errors.Is() instead of
os.IsNotExist() [4].

The Go implementation of Toolbx never used any Go older than 1.13 [5],
and currently it requires Go >= 1.22 [6].  So, there's no reason not to
use the more modern and recommended alternative.

[1] https://pkg.go.dev/os#IsNotExist

[2] https://pkg.go.dev/errors#Is

[3] https://go.dev/blog/go1.13-errors

[4] Go commit b641f0dcf48aa748
    https://github.com/golang/go/commit/b641f0dcf48aa748
    https://github.com/golang/go/issues/41122

[5] Commit d857471aa2
    https://github.com/containers/toolbox/commit/d857471aa2f233e5
    https://github.com/containers/toolbox/pull/318

[6] Commit eb73692618
    https://github.com/containers/toolbox/commit/eb736926183b1c20
    https://github.com/containers/toolbox/pull/1662

https://github.com/containers/toolbox/pull/1691

src/cmd/initContainer.go

@@ -944,7 +944,7 @@ func redirectPath(containerPath, target string, folder bool) error {
 
 	err := os.Remove(containerPath)
 	if folder {
-		if err != nil && !os.IsNotExist(err) {
+		if err != nil && !errors.Is(err, os.ErrNotExist) {
 			return fmt.Errorf("failed to redirect %s to %s: %w", containerPath, target, err)
 		}