FIX Allow patch tag workflow to be dispatched correctly.

[GuySartorelli]

Two changes were made here:

1. Don't check for the presence of the file up front. Turns out the actions/checkout action is per-job. So we'd have to run that again in this job for it to work. Instead, we can just check the response code of the request, and if it's a 404 we know the file wasn't there.
2. Add the actions:write permission. This is required, or else we get a 403 response code.

I have tested this in a repo I control, and found both of these changes are necessary for it to work as expected.

I have also pulled this fork in a repo I control to check and the new permission does need to be added in ci.yml for community members: https://github.com/GuySartorelli/silverstripe-composable-validators/actions/runs/10275469540
I've added back the warning from #139 (comment) as a result.

Issue

• Fails to dispatch tag-patch-release.yml #145