Configurable pipeline upload retry attempts

**Is your feature request related to a problem? Please describe.**

When running `buildkite-agent pipeline upload`, the CLI will attempt a hard-coded 60 attempts over 5 minutes to upload the pipeline. We personally would like to fail faster than that, but there is currently no way to configure this.

[The upload code in question](https://github.com/buildkite/agent/blob/31ab70250f3f5abe0818f48c9d5176bc5ac3f122/agent/pipeline_uploader.go#L179)
[
The hard-coded constants](https://github.com/buildkite/agent/blob/31ab70250f3f5abe0818f48c9d5176bc5ac3f122/agent/pipeline_uploader.go#L20)

**Describe the solution you'd like**

Some way of configuring the retry parameters, probably via flags

**Describe alternatives you've considered**

N/A

**Additional context**

N/A