Fix e2e test, which was broken due to helm4 upgrade

[inteon]

For Helm 4, we have to pass the --verify=false flag (see https://github.com/databus23/helm-diff/blob/0a397b436a2a676c3cce98a9b3bb21a592e09859/README.md?plain=1#L29).

This causes the e2e test to fail (eg. https://github.com/jetstack/jetstack-secure/actions/runs/21257026671/job/61174350691?pr=758).

[wallrj-cyberark]

Thanks @inteon

I tested it locally and the fix seems to work well

richard@localhost:~/projects/jetstack/jetstack-secure$ make helm-plugins
>>> Installing helm-diff plugin
Error: plugin source does not support verification. Use --verify=false to skip verification
make: *** [make/02_mod.mk:72: helm-plugins] Error 1


richard@localhost:~/projects/jetstack/jetstack-secure$ gh pr checkout 759
branch 'helm4_e2e_fix' set up to track 'origin/helm4_e2e_fix'.
Switched to a new branch 'helm4_e2e_fix'


richard@localhost:~/projects/jetstack/jetstack-secure$ make helm-plugins
>>> Installing helm-diff plugin
WARNING: Skipping plugin signature verification
Downloading https://github.com/databus23/helm-diff/releases/download/v3.9.8/helm-diff-linux-amd64.tgz
Preparing to install into /home/richard/.local/share/helm/plugins/helm-diff
Installed plugin: diff

Merge when the E2E tests pass.