Option NOT To Sign The APK Even If Debug Keystore Is Present

**Is your feature request related to a problem? Please describe.**
I'd like to generate unsigned APK using `build-apks` command.

**Describe the solution you'd like**
A flag switch `--no-sign` that won't sign the APK even if debug keystore is present. Of course this is mutually exclusive with `--ks` option.

**Describe alternatives you've considered**
My current workaround is to rename `debug.keystore` to something else before running bundletool and rename it to their original name afterwards.

**Additional context**
I'd like to distribute **unsigned** APK. The APKs are meant to be modified further by end-users and signed by themselves.