Use pipenv in the action to install dependencies

This provides a single source of truth for the dependencies for
the project.

action.yml

@@ -52,9 +52,15 @@ runs:
       INPUTS_ISSUE_END_STATE: ${{ inputs.issue_end_state }}
       INPUTS_ISSUE_REOPEN_STATE: ${{ inputs.issue_reopen_state }}
     run: |
-      pip3 install -r requirements.txt
+      # Install pipenv to a temporary directory
+      # We use a temporary directory because the action didn't previously have a requirement for
+      # pipenv to be installed, and we don't want to pollute the user's existing pip installations.
+      pip3 install pipenv --target "$RUNNER_TEMP/pip"
+      # Run pipenv from the temporary directory
+      $RUNNER_TEMP/pip/pipenv install
       REPOSITORY_NAME="$(echo "$GITHUB_REPOSITORY" | cut -d/ -f 2)"
-      ./gh2jira sync \
+      # Run pipenv from the temporary directory
+      $RUNNER_TEMP/pip/pipenv run ./gh2jira sync \
         --gh-url "$GITHUB_API_URL" \
         --gh-token "$INPUTS_GITHUB_TOKEN" \
         --gh-org "$GITHUB_REPOSITORY_OWNER" \