feat: add `GO_COBERTURA_FLAGS` env var for `gocover-cobertura`

Presentation

gocover-cobertura does not support go build tags, a pull request exists about it and a proposition given to solve this issue is to use GOFLAGS when running it. This adds a new variable to use it.

Closes #30 (closed)

Checklist

• General:
  • use rules instead of only/except
  • optimized cache configuration (wherever applicable)
• Publicly usable:
  • untagged runners
  • no proxy configuration but support http_proxy/https_proxy/no_proxy
  • no custom CA certificate(s) but supports $CUSTOM_CA_CERTS or $DEFAULT_CA_CERTS to declare custom CA certificate(s)
  • internet hostnames/urls only
• Used Docker images:
  • public images
  • official images (when possible)
  • latest tag (when possible)
• Documented:
  • README.md documents the new feature
  • kicker.json describes the new feature
• Tested & examplified:
  • (url to a project sample successfully using the new feature).