fix: validate backport branch name

[knqyf263]

Description

• validate the backport workflow’s extracted branch name before exporting to GITHUB_ENV
• reject empty or unsafe names while allowing dotted release branches like release/v0.53.1
• verified on knqyf263/trivy#69 that safe names proceed (workflow run 18117590123) and unsafe names abort immediately (run 18117630437 and 18121474365)

Checklist

• I've read the guidelines for contributing to this repository.
• I've followed the conventions in the PR title.
• I've added tests that prove my fix is effective or that my feature works.
• I've updated the documentation with the relevant information (if needed).
• I've added usage information (if the PR introduces new options)
• I've included a "before" and "after" example to the description (if the PR is a user interface change).

[Copilot]

Pull Request Overview

This PR enhances security by validating backport branch names in the GitHub workflow before setting them as environment variables. The validation prevents potential command injection by ensuring branch names contain only safe characters while still allowing dotted release branches like release/v0.53.1.

• Added validation for extracted branch names using regex pattern matching
• Added error handling to exit immediately when invalid or empty branch names are detected
• Improved shell scripting practices with proper variable quoting

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[Copilot]

The regex pattern allows dots which could enable path traversal attacks with patterns like '../'. Consider restricting dots to only appear in specific contexts (e.g., version numbers) or use a more restrictive pattern like ^[A-Za-z0-9._-]+(/[A-Za-z0-9._-]+)*$ to prevent directory traversal.

Suggested change

	if [[ -z "$BRANCH_NAME" || ! "$BRANCH_NAME" =~ ^[A-Za-z0-9._/-]+$ ]]; then
	if [[ -z "$BRANCH_NAME" || ! "$BRANCH_NAME" =~ ^[A-Za-z0-9._-]+(/[A-Za-z0-9._-]+)*$ ]]; then

[knqyf263]

Improved in e0ff47b