doc: add 'git clean -xfd' to backport guide

doc: add 'git clean -xfd' to backport guide · nodejs/node@62ea82b

Original file line number	Diff line number	Diff line change
`@@ -44,6 +44,11 @@ cd $NODE_DIR`
`44`	`44`	`git fetch upstream v6.x-staging:v6.x-staging -f`
`45`	`45`	`# Assume we want to backport PR #10157`
`46`	`46`	`git checkout -b backport-10157-to-v6.x v6.x-staging`
	`47`	`+# Ensure there are no test artifacts from previous builds`
	`48`	`+# Note that this command deletes all files and directories`
	`49`	`+# not under revision control below the ./test directory.`
	`50`	`+# It is optional and should be used with caution.`
	`51`	`+git clean -xfd ./test/`
`47`	`52`	```
`48`	`53`
`49`	`54`	`4. After creating the branch, apply the changes to the branch. The cherry-pick`