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]

Codecov Report

All modified and coverable lines are covered by tests ✅

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

Additional details and impacted files

@@            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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