nginx: add test matrix, release builds

[seemk]

• Use only Docker for build and tests. This removed the redundant build step and allows for easier caching.
• Use only release builds for testing.
• Add more distros (Ubuntu 18.04, 20.10) along with mainline nginx (1.19.x) to the testing matrix.
• Add Docker layer caching via buildx + Github Actions cache. Shaves off 15 minutes of build time from grpc on every distro.
• Provide prebuilt libraries / artifacts for each distro and nginx configuration.
• Add CI push trigger paths to nginx.yml so nginx builds won't activate on unrelated PRs.
• Fix trace.json output file permissions for tests.

[seemk]

There, good to go now. Tests found an issue with the default nginx on Ubuntu 18.04: the package manager's nginx wasn't compiled with compatibility flags.