rpk: improved err msg in rpk tune --output-script

If the user used a directory it would fail without
giving any meaningful error.

Manual Backport of PR redpanda-data#13473

src/go/rpk/pkg/cli/cmd/redpanda/tune/tune.go

@@ -101,6 +101,11 @@ To learn more about a tuner, run 'rpk redpanda tune help <tuner name>'.
 				tunerFactory = factory.NewScriptRenderingTunersFactory(
 					fs, *cfg, outTuneScriptFile, timeout)
 			} else {
+				isDir, err := afero.IsDir(fs, outTuneScriptFile)
+				out.MaybeDie(err, "unable to check if %q is a dir or a file: %v", outTuneScriptFile, err)
+				if isDir {
+					out.Die("please use a filename instead of a directory name in --output-script")
+				}
 				tunerFactory = factory.NewDirectExecutorTunersFactory(
 					fs, *cfg, timeout)
 			}