GHA/http3-linux: fix nghttpx build and other tweaks

GHA/http3-linux: fix nghttpx build and other tweaks

- fix `nghttp2` build to also build the `nghttpx` application.
  Restore required `libc-ares-dev`. Also confirm that `libev-dev` is
  required too. Document these requirements.
  Follow-up to 0455d8772a1af20ce63c46c5738582aa9b1b8441 #18509

- explicitly enable `nghttpx` for the `nghttp2` build to make it fail if
  requirements aren't met:
  ```
  configure: error: applications were requested (--enable-app) but dependencies are not met.
  ```

- explicitly install brotli, zstd, zlib for the dependency builds.
  Of these, zstd and zlib are preinstalled. zlib is required for
  `nghttpx`. zstd and brotli doesn't seem to be used, but keep them
  there just in case and to match the test env.
  Follow-up to 0455d8772a1af20ce63c46c5738582aa9b1b8441 #18509

- enable brotli for `nghttpx`. It doesn't change the tests, and also
  cost almost nothing, so I figure why not.

Closes #18522