build: GitHub Actions badge

[kevgo]

Updates the build status badge to show the status of GitHub Actions.

Preview: https://github.com/kevgo/ory-oathkeeper/blob/kg-badge/README.md

Please also remove the CircleCI jobs from the list of required status checks in the settings for this repo.

Checklist

• I have read the contributing guidelines.
• I have referenced an issue containing the design document if my change
  introduces a new feature.
• I am following the
  contributing code guidelines.
• I have read the security policy.
• I confirm that this pull request does not address a security
  vulnerability. If this pull request addresses a security. vulnerability, I
  confirm that I got green light (please contact
  security@ory.sh) from the maintainers to push
  the changes.
• I have added tests that prove my fix is effective or that my feature
  works.
• I have added or changed the documentation.

[codecov]

Codecov Report

Merging #979 (c2c587d) into master (f96f2be) will not change coverage.
The diff coverage is n/a.

❗ Current head c2c587d differs from pull request most recent head b404c70. Consider uploading reports for the commit b404c70 to get more accurate results

@@           Coverage Diff           @@
##           master     #979   +/-   ##
=======================================
  Coverage   66.26%   66.26%           
=======================================
  Files         107      107           
  Lines        4764     4764           
=======================================
  Hits         3157     3157           
  Misses       1325     1325           
  Partials      282      282           

Impacted Files	Coverage Δ
internal/httpclient/models/generic_error.go	0.00% <ø> (ø)
...ernal/httpclient/models/health_not_ready_status.go	0.00% <ø> (ø)
internal/httpclient/models/health_status.go	0.00% <ø> (ø)
internal/httpclient/models/json_web_key.go	0.00% <ø> (ø)
internal/httpclient/models/json_web_key_set.go	0.00% <ø> (ø)
internal/httpclient/models/rule.go	0.00% <ø> (ø)
internal/httpclient/models/rule_handler.go	0.00% <ø> (ø)
internal/httpclient/models/rule_match.go	0.00% <ø> (ø)
internal/httpclient/models/upstream.go	0.00% <ø> (ø)
internal/httpclient/models/version.go	0.00% <ø> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 628f50e...b404c70. Read the comment docs.