Merge pull request #91 from crazy-max/pr_target_event

Handle `pull_request_target` event

__tests__/fixtures/event_pull_request_target.env

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+GITHUB_ACTION=crazy-maxghaction-dump-context
+GITHUB_ACTIONS=true
+GITHUB_ACTION_PATH=/home/runner/work/_actions/crazy-max/ghaction-dump-context/v1
+GITHUB_ACTION_REF=
+GITHUB_ACTION_REPOSITORY=
+GITHUB_ACTOR=crazy-max
+GITHUB_API_URL=https://api.github.com
+GITHUB_BASE_REF=master
+GITHUB_ENV=/home/runner/work/_temp/_runner_file_commands/set_env_cc4da131-0660-4771-8663-0f96c0fc944f
+GITHUB_EVENT_NAME=pull_request_target
+GITHUB_EVENT_PATH=./__tests__/fixtures/event_pull_request_target.json
+GITHUB_GRAPHQL_URL=https://api.github.com/graphql
+GITHUB_HEAD_REF=test-pr-target
+GITHUB_JOB=build
+GITHUB_PATH=/home/runner/work/_temp/_runner_file_commands/add_path_cc4da131-0660-4771-8663-0f96c0fc944f
+GITHUB_REF=refs/heads/master
+GITHUB_REPOSITORY=crazy-max/test-docker-action
+GITHUB_REPOSITORY_OWNER=crazy-max
+GITHUB_RETENTION_DAYS=90
+GITHUB_RUN_ID=867725119
+GITHUB_RUN_NUMBER=12
+GITHUB_SERVER_URL=https://github.com
+GITHUB_SHA=f24900bfcfee76d8055c8421a164c7e57ad20e68
+GITHUB_WORKFLOW=push-ghcr
+GITHUB_WORKSPACE=/home/runner/work/test-docker-action/test-docker-action