bindgen does not used the `.bazelrc` configured `cxxopts`

In https://github.com/lowRISC/opentitan/pull/14860, I found that while building the dependencies for my bindgen rule, the compiler was not being invoked with the project-specified `--cxxopt`s. In the CI environment, this resulted in the compiler being invoked with `-std=c++0x` rather than the project specified `-std=gnu++14`.

In order to overcome this issue, I added the following to the `.bazelrc`:
```
build --action_env=BAZEL_CXXOPTS="-std=gnu++14"
```