Fix Issue 20408 - style checker should not check backup files

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	20408	normal	style checker should not check backup files

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub fetch digger
dub run digger -- build "master + phobos#7289"