Update pipeline.rb to support multiple pipeline uploads

[gmogames]

Summary

This PR fixes an issue that prevented multiple buildkite-builder run commands from being executed sequentially within the same container/process, enabling more efficient CI/CD workflows.

Problem

Previously, running multiple buildkite-builder run commands in sequence (e.g., via && chaining) would fail because the first run would set job metadata that couldn't be overwritten by subsequent runs, causing the error:
Error: Could not set job meta-data: The job meta-data key "buildkite_builder_uploaded_[pipeline_name]" already exists

Solution

• Modified metadata handling to allow multiple pipeline uploads within the same container lifecycle
• Each pipeline upload now properly manages its own metadata without conflicts
• Maintains backward compatibility with single pipeline usage

Usage

Before (would fail)

# This would fail on the second command
buildkite-builder run pipeline.deploy && 
buildkite-builder run pipeline.unit.tests

After (now works)

# Multiple pipelines can now be uploaded sequentially
buildkite-builder run pipeline.deploy && \
buildkite-builder run pipeline.unit.tests && \
buildkite-builder run pipeline.integration

Benefits

• Container Efficiency: Allows multiple pipelines to be uploaded from a single container
• Performance: Eliminates the need to spin up separate containers for each pipeline
• Flexibility: Enables complex CI/CD workflows with conditional pipeline uploads

Testing

✅ Tested sequential pipeline uploads within same container
✅ Verified backward compatibility with single pipeline usage
✅ Confirmed metadata handling doesn't cause conflicts

Impact

• Reduces CI/CD execution time by avoiding multiple container startups
• Enables more complex pipeline orchestration patterns
• Fully backward compatible with existing single-pipeline workflows