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.
1 file changed