Update cli/urfave to v2

The library that we use to have a nice cli, [urfave/cli](https://github.com/urfave/cli) has had a new major version, v2, for a little while now. For the sake of staying as current as possible with our dependencies, we should update this to the newest version.

Unfortunately, v2 of urfave/cli includes [a breaking change](https://github.com/urfave/cli/blob/master/docs/migrate-v1-to-v2.md#flags-before-args=) where now, arguments to functions must come before flags - ie, where
```
buildkite-agent pipeline upload /path/to/pipeline --replace
```
is valid in v1, in v2 only
```
buildkite-agent pipeline upload --replace /path/to/pipeline
```
is allowed.

This issue is here to track that process, and note that we might want to (but won't necessarily) upgrade to the latest version of cli in agent v4.

Start with the urfave/cli v1-to-v2 migration guide linked in the issue, then inspect the pipeline upload command and the other agent CLI entry points. Verify that the dependency is upgraded and that arguments before flags, such as `pipeline upload --replace /path/to/pipeline`, work as required by v2; consider whether the change belongs in agent v4.

Written by the indexing model from the issue text.