Change pr-labeler workflow trigger to pull_request_target

[nowsprinting]

When PR from public fork, Can not access configuration file in pull_request trigger.
e.g., https://github.com/DeNA/Anjin/actions/runs/4633726453

pull_request_target trigger is:

• GITHUB_TOKEN is granted read/write repository permission unless the permissions key is specified and the workflow can access secrets, even when it is triggered from a fork
• Runs in the context of the base of the pull request (NOT merge commit)

Refs:

• Action doesn't work for forks (Resource not accessible by integration) TimonVS/pr-labeler-action#25 (comment)
• https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows#pull_request_target

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[Kesin11]

LGTM

refs for other @DeNA/swet member

https://madogiwa0124.hatenablog.com/entry/2022/05/29/110148

forkされたリポジトリからのPRの場合、GITHUB_TOKENが読み取り専用の権限となりsecretにもアクセス出来ない。

https://pankona.github.io/blog/2021/03/29/github-actions-pull-request-target/

これで何が困るかというと、例えば fork 元で設定している secrets にアクセスできないという点。secrets にアクセスできないということは、たとえば普通 secrets に入れておいて github actions で用いたくなるような GITHUB_TOKEN だとか SLACK_TOKEN だとか、あるいは FIREBASE_SERVICE_ACCOUNT_XXX のようなものだとかが参照できないということが起こる。

[nowsprinting]

Thx, @Kesin11 san!