fix: handle CNI config stat errors without panic

[immanuwell]

Small fix for a CNI config path footgun.

If --cni-netconfpath / $NETCONFPATH points at a regular file, fsExists gets a stat error and then derefs a nil FileInfo. Boom: panic. This can happen from a bad local path, no weird infra limit needed.

Repro:

1. Add TestFSExistsPropagatesStatError from this PR on main.
2. Run go test ./pkg/netutil -run TestFSExistsPropagatesStatError -count=1.
3. It panics at pkg/netutil/store.go:56.

This returns the stat error instead, and keeps missing-file behavior intact.

Tests:

• go test ./pkg/netutil -run TestFSExistsPropagatesStatError -count=1
• go test ./pkg/netutil -count=1
• go test ./pkg/...

[haytok]

Not sure if the scenario covered by TestFSExistsPropagatesStatError can happen via --cni-netconfpath from the CLI, but if os.Stat returns a non-NotExist error the current code panics on the nil, and returning the error instead is obviously safe. LGTM, Thanks!