fix: add trailing slash to repo path when listing objects

[nerdeveloper]

This pull request includes a minor update to the fetchChartsInStorage function in the pkg/chartmuseum/server/multitenant/cache.go file. The change ensures that the repo parameter is joined with an empty string using pathutil.Join when calling the ListObjects method on StorageBackend.

To make it super clear:

Before Fix:
└── bucket
    ├── abc-def-1.1.1.tgz    ❌ (ChartMuseum wrongly included these)
    ├── abc-def-1.1.24.tgz   ❌ (because they start with "abc")
    └── abc/
        └── xyz-1.1.1.tgz    ✅ (should only include this)

After Fix:
└── bucket
    ├── abc-def-1.1.1.tgz    ✅ (ChartMuseum correctly ignores these)
    ├── abc-def-1.1.24.tgz   ✅ (because they're not in the abc/ folder)
    └── abc/
        └── xyz-1.1.1.tgz    ✅ (only includes this, as it should!)

[nerdeveloper]

i messed up my commits trying to verify commits