CCM-11938: Fix manual workflow

[sidnhs]

Description

Fix manual worfklow for proxygen, adding an input to build sandbox container if needed

When I didn't create a PR: https://github.com/NHSDigital/nhs-notify-supplier-api/actions/runs/18344058066

After I created a PR: https://github.com/NHSDigital/nhs-notify-supplier-api/actions/runs/18344215119

After I create a PR and ticked the build sandbox flag: https://github.com/NHSDigital/nhs-notify-supplier-api/actions/runs/18344308039

Type of changes

• Refactoring (non-breaking change)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would change existing functionality)
• Bug fix (non-breaking change which fixes an issue)

Checklist

• I am familiar with the contributing guidelines
• I have followed the code style of the project
• I have added tests to cover my changes
• I have updated the documentation accordingly
• This PR is a result of pair or mob programming

---

Sensitive Information Declaration

To ensure the utmost confidentiality and protect your and others privacy, we kindly ask you to NOT including PII (Personal Identifiable Information) / PID (Personal Identifiable Data) or any other sensitive data in this PR (Pull Request) and the codebase changes. We will remove any PR that do contain any sensitive information. We really appreciate your cooperation in this matter.

• I confirm that neither PII/PID nor sensitive data are included in this PR and the codebase changes.