Add --warning-suppression-mappings to cgoAbsEnvFlags (#4448) **What type of PR is this?** Bug fix **What does this PR do? Why is it needed?** Since `--warning-suppression-mappings` points to a file path, it doesn't work with the additional sandboxing go introduces. This change adds the flag to `cgoAbsEnvFlags` to allow it to work during cgo compilation.