Attempt to fix labeler permissions

[falbrechtskirchinger]

Change trigger to pull_request_target for write permissions and limit permissions to minimum required.

[nlohmann]

The job seems not to be executed.

[falbrechtskirchinger]

I don't think this will trigger until it's merged.

Testing here. I've pushed it to develop and submitted a PR. (Of course, we have the same issue of not being able to test permissions this way.)
falbrechtskirchinger#1

[falbrechtskirchinger]

All I can say is that it runs (once merged). If it improves the situation over the status-quo of not labeling PRs, I don't know.

[coveralls]

Coverage remained the same at 100.0% when pulling 12819cf on falbrechtskirchinger:labeler-permissions into 500bdc5 on nlohmann:develop.

[nlohmann]

Let's give it a try!

[nlohmann]

Looks good to me.