**Issue:**

The "std-compat" CI isn't doing what it's supposed to: building aws-crt-cpp with different versions of the C++ std.

It broke accidentally when [this commit](eaaaa60) stopped passing `--env CXXFLAGS` to the container.

**Description of changes:**

Fix it, so C++ version is passed to the container.

Set it via `CMAKE_CXX_STANDARD=11`, instead of via `CXXFLAGS=-std=c++11`, since this is the "more correct" way to tell CMake the C++ standard you want. Doing it via `CXXFLAGS=-std=c++11` can (depending on the version of CMake you're using) force compiler extensions to be disabled.

Update submodules, bringing in this notable fix:
- awslabs/aws-c-io#718

```
aws-c-auth         v0.8.6 -> v0.9.0
aws-c-cal          v0.8.7 -> v0.8.8
aws-c-common       v0.12.0 -> v0.12.2
aws-c-io           v0.17.0 -> v0.17.1
aws-checksums      v0.2.3 -> v0.2.5
aws-lc             v1.48.4 -> v1.48.5
s2n                v1.5.14 -> v1.5.15
```

Co-authored-by: GitHub Actions <aws-sdk-common-runtime@amazon.com>