Downstream checks: add show closed and only failed options

[VenelinMartinov]

This adds a flag to print closed downstream check PRs' status.

Useful if pulumibot auto-closed failed PRs.

It also adds an --only-failed flag to only print failed checks.

Example:

``` > python scripts/downstream_checks.py --hash 7a5e480 --show-closed --only-failed FAILURE https://github.com/pulumi/pulumi-cloudflare/pull/784 FAILURE https://github.com/pulumi/pulumi-aws/pull/3983 FAILURE https://github.com/pulumi/pulumi-gcp/pull/2023 FAILURE https://github.com/pulumi/pulumi-auth0/pull/547 FAILURE https://github.com/pulumi/pulumi-okta/pull/624 FAILURE https://github.com/pulumi/pulumi-gitlab/pull/614 FAILURE https://github.com/pulumi/pulumi-github/pull/669 FAILURE https://github.com/pulumi/pulumi-azuread/pull/1081 FAILURE https://github.com/pulumi/pulumi-azure/pull/2065 MISSING https://github.com/pulumi/pulumi-meraki/actions/workflows/upgrade-bridge.yml MISSING https://github.com/pulumi/pulumi-rabbitmq/actions/workflows/upgrade-bridge.yml MISSING https://github.com/pulumi/pulumi-rancher2/actions/workflows/upgrade-bridge.yml MISSING https://github.com/pulumi/pulumi-openstack/actions/workflows/upgrade-bridge.yml MISSING https://github.com/pulumi/pulumi-postgresql/actions/workflows/upgrade-bridge.yml ```

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 61.03%. Comparing base (b3c8151) to head (c2d2617).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #2033 +/- ## ========================================== - Coverage 61.03% 61.03% -0.01% ========================================== Files 333 333 Lines 44861 44861 ========================================== - Hits 27381 27379 -2 - Misses 15961 15962 +1 - Partials 1519 1520 +1 ```

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