fix: use branch name directly

[edx-requirements-bot]

$default-branch is only available in workflow-templates so we'll use branch name directly in this workflow

https://stackoverflow.com/questions/64781462/github-actions-default-branch-variable

Additional information from script execution

Python code cleanup by the cleanup-python-code Jenkins job.

This pull request was generated by the cleanup-python-code Jenkins job, which ran

cd .github/workflows;  TEAM_REVIEWERS=$(grep 'team_reviewers:' upgrade-python-requirements.yml | awk '{print $2}'); TEAM_REVIEWERS=${TEAM_REVIEWERS/--team-reviewers=/}; SCHEDULE=$(grep 'cron:' upgrade-python-requirements.yml -m 1 | sed 's/^.*: //');   EMAIL_TO=$(grep 'email_address: ' upgrade-python-requirements.yml -m 1 | sed 's/^.*: //'); SEND_SUCCESS_NOTIFICATION=$(echo false); : > upgrade-python-requirements.yml; printf "name: Upgrade Requirements\n\non:\n  schedule:\n     - cron: ${SCHEDULE}\n  workflow_dispatch:\n     inputs:\n       branch:\n         description: 'Target branch to create requirements PR against'\n         required: true\n         default: 'master'\njobs:\n   call-upgrade-python-requirements-workflow:\n    with:\n       branch: \${{ github.event.inputs.branch }}\n       team_reviewers: ${TEAM_REVIEWERS}\n       email_address: ${EMAIL_TO}\n       send_success_notification: ${SEND_SUCCESS_NOTIFICATION}\n    secrets:\n       requirements_bot_github_token: \${{ secrets.REQUIREMENTS_BOT_GITHUB_TOKEN }}\n       requirements_bot_github_email: \${{ secrets.REQUIREMENTS_BOT_GITHUB_EMAIL }}\n       edx_smtp_username: \${{ secrets.EDX_SMTP_USERNAME }}\n       edx_smtp_password: \${{ secrets.EDX_SMTP_PASSWORD }}\n    uses: edx/.github/.github/workflows/upgrade-python-requirements.yml@master\n" >> upgrade-python-requirements.yml;

The following packages were installed:
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