fix: avoid re-mounting GCR volume with credentials

[afdesk]

Description

This PR adds a validation to ensure that a volume containing GCR credentials is present before proceeding.
If such a volume is already mounted, the logic prevents redundant re-mounting to avoid duplication and potential conflicts.

Related issues

• Close A vulnerabilityreport scan job cannot be started due to volume name duplicates in GKE #2407

Checklist

• I've read the guidelines for contributing to this repository.
• I've added tests that prove my fix is effective or that my feature works.
• I've updated the documentation with the relevant information (if needed).
• I've added usage information (if the PR introduces new options)
• I've included a "before" and "after" example to the description (if the PR is a user interface change).

[afdesk]

@simar7 I remember you have a meticulous attitude toward testing, and I was thinking about how to add tests here.

A simple table-driven test for createEnvAndVolumeForGcr doesn’t make much sense in this context.

Once we resolve issues #2595 and #2769, we’ll be able to add a more meaningful test that covers multiple containers.

[simar7]

To test something like this where the number of invocations that happen is what we're ultimately interested in, we could add a test where we keep track of invocations. Knowing how many times we expect createEnvandVolumeForGcr to get called can let us know if the logic is working as expected or if there's a bug.

In this example, with an early return in case of an existing mount, we should have less invocations than before introducing this change.

[simar7]

@simar7 I remember you have a meticulous attitude toward testing, and I was thinking about how to add tests here.

A simple table-driven test for createEnvAndVolumeForGcr doesn’t make much sense in this context.

Once we resolve issues #2595 and #2769, we’ll be able to add a more meaningful test that covers multiple containers.

I left an idea https://github.com/aquasecurity/trivy-operator/pull/2801/files#r2591586352