fix(internal/gensupport): update shouldRetry for GCS uploads

[tritone]

A few fixes to shouldRetry, which is used only for storage uploads.

• Remove attempt to unwrap syscall errors and use string matching instead, as is used in the veneer client layer for storage.
• Use errors.Is and appropriate sentinel for net.ErrClosed.
• Add broken pipe error, which is causing flakes in veneer layer tests (see googleapis/google-cloud-go#10374
• Add unit test for shouldRetry.

Retry conformance tests in google-cloud-go/storage pass.

Closes googleapis/google-cloud-go#9178