cli: use the current Pulumi stack's environments by default

[AaronFriel]

When run in a Pulumi project, the esc and pulumi env CLI commands would be much more useful if they env were optional and used the currently selected stack's environments by default.

This is useful for scenarios such as in CI/CD infrastructure, such as Pulumi Deployments and GitHub Actions, in which a user may want to pulumi env run -- ... some command prior to running pulumi up.

For CI/CD configuration especially, this is useful as the environment can be configured per-stack, and it would be challenging to ensure that a pre-up command used the same configuration as the Pulumi stack.

With this feature, users will not need to keep in sync their IaC configuration (Pulumi.*.yaml) and their CI/CD configuration (pre-run commands, GitHub workflow files) in sync.