Enable autofixes on forks

[72636c]

If this is possible, I think we'd need cleverer branch inference to target e.g. refs/pull/123/head.

Failed to push fix commit.
Does your CI environment have write access to your Git repository?

GraphqlResponseError: Request failed due to following response errors:

Branch not found
   at /workdir/node_modules/@octokit/graphql/dist-node/index.js:81:13
   at processTicksAndRejections (node:internal/process/task_queues:96:5)
   at async uploadFileChanges (/workdir/node_modules/skuba/lib/api/github/push.js:128:18)
   at async Object.uploadAllFileChanges (/workdir/node_modules/skuba/lib/api/github/push.js:48:20)
   at async withTimeout (/workdir/node_modules/skuba/lib/utils/wait.js:45:12)
   at async throwOnTimeout (/workdir/node_modules/skuba/lib/utils/wait.js:36:18)
   at async autofix (/workdir/node_modules/skuba/lib/cli/lint/autofix.js:106:17)
   at async externalLint (/workdir/node_modules/skuba/lib/cli/lint/external.js:105:3)
   at async lint (/workdir/node_modules/skuba/lib/cli/lint/index.js:37:3)
{
  request: {
    query: '\n' +
      '      mutation Mutation($input: CreateCommitOnBranchInput!) {\n' +
      '        createCommitOnBranch(input: $input) {\n' +
      '          commit {\n' +
      '            oid\n' +
      '          }\n' +
      '        }\n' +
      '      }\n' +
      '    ',
    variables: { input: [Object] },
    headers: {...}
  },
  headers: {...},
  response: { data: { createCommitOnBranch: null }, errors: [ [Object] ] },
  errors: [
    {
      type: 'NOT_FOUND',
      path: [Array],
      locations: [Array],
      message: 'Branch not found'
    }
  ],
  data: { createCommitOnBranch: null }
}