internal: run goimports on generated snippets

[codyoss]

I believe we should be able to run imports.Process. Currently unformatted imports fail CI on regen. Manually patched in #4025

[codyoss]

cc @tbpg

[tbpg]

goimports is run on generated snippets here:

google-cloud-go/internal/gapicgen/gensnippets/gensnippets.go

Lines 82 to 88 in a297278

	if len(dirs) > 0 {
	cmd := execabs.Command("goimports", "-w", ".")
	cmd.Dir = outDir
	if err := cmd.Run(); err != nil {
	return fmt.Errorf("failed to run goimports: %v", err)
	}
	}