SinkBinding eliminates all volumes on Deployment subject

**Describe the bug**
Given the subject in a SinkBinding is an arbitrary Deployment that brings its own volumes/volumeMounts spec these get eliminated as soon as the SinkBinding has injected K_SINK envVars. As a result the initial volume/volumeMount information is lost on the Deployment.

**Expected behavior**
The SinkBinding should preserve existing volume/volumeMount specs on the Deployment subject.

**To Reproduce**
* Create a Deployment with a custom volume/volumeMount
* Create a SinkBinding resource with the given Deployment as a subject
* See the existing volume/volumeMount disappear from the Deployment spec as soon as the SinkBinding envVar injection is done

**Knative release version**
v1.14.0

**Additional context**
used kubectl apply -f to create both Deployement and SinkBinding. Tested on local Kind cluster.

Search the repository for SinkBinding and the K_SINK environment-variable injection path, then reproduce the issue with a Deployment containing a custom volume and volumeMount. Done means the existing volume and volumeMount specifications remain after the SinkBinding injection.

Written by the indexing model from the issue text.