fix check order in setGithubToken

[sarisia]

In the current version, when trying to deploy to an external repository with
github_token, it fails with an inappropriate message.

Run workflow below:

jobs:
  deploy:
    runs-on: ubuntu-latest
    steps:
      # omitted
      - run: hugo
      - uses: peaceiris/actions-gh-pages@v3
        with:
          github_token: ${{ secrets.GITHUB_TOKEN }}
          external_repository: sarisia/external-repository
          publish_branch: master
          publish_dir: ./public

and get:

Setup auth token
  [INFO] setup GITHUB_TOKEN
  ##[error]Action failed with "You deploy from master to master"

I think this is inappropriate and difficult to identify the cause of failure, so
this PR changes check order in setGithubToken() to make error message more
intelligible.

[codecov]

Codecov Report

Merging #382 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #382   +/-   ##
=======================================
  Coverage   58.33%   58.33%           
=======================================
  Files           4        4           
  Lines         240      240           
  Branches       43       43           
=======================================
  Hits          140      140           
  Misses        100      100           

Impacted Files	Coverage Δ
src/set-tokens.ts	43.05% <100.00%> (ø)

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[peaceiris]

Great! Thanks