fix: Don't try to revoke a token that has already expired

[wwsean08]

This is a PR to fix when a CI job fails because it tries to revoke a token which has already expired. Given that github provides the expiration time, it's worth referencing that and preventing the error in the first place.

Testing

This was tested using an internal repostiory, below are the workflow files and the results:

Skipping revoke as it's expired

name: Testing app rejection update
on:
  push:
    branches:
      - sean-smith-test

jobs:
  test:
    runs-on: ubuntu-latest
    steps:
      - name: checkout code
        uses: actions/checkout@v4
      - name: Setup app token
        uses: wwsean08/github-app-token@v2
        with:
          app_id: ${{ secrets.APP_READ_ALL_REPOS_ID }}
          private_key: ${{ secrets.APP_READ_ALL_REPOS_KEY_B64 }}
      - name: Sleep for about an hour
        run: sleep 61m

Revoking as it's not expired

name: Testing app rejection update
on:
  push:
    branches:
      - sean-smith-test

jobs:
  test:
    runs-on: ubuntu-latest
    steps:
      - name: checkout code
        uses: actions/checkout@v4
      - name: Setup app token
        uses: wwsean08/github-app-token@v2
        with:
          app_id: ${{ secrets.APP_READ_ALL_REPOS_ID }}
          private_key: ${{ secrets.APP_READ_ALL_REPOS_KEY_B64 }}
      - name: Sleep for about an hour
        run: sleep 5

[PoltoS]

Closes #111