Fix an issue from golangci-lint

[sanposhiho]

This PR fixes an issue from golangci-lint.

(*'-') < make verify-golangci-lint

...

pkg/repo/repo.go:270:2: Consider preallocating `allKEPs` (prealloc)
        var allKEPs []*api.Proposal
        ^
make: *** [verify-golangci-lint] Error 1

And I think we can enable golangci-lint in prow-job. It's skipped currently.
https://github.com/kubernetes/enhancements/blob/master/hack/verify.sh#L40

/cc @justaugustus

[mkimuram]

@justaugustus I confirmed that make verify-golangci-lint causes the error and this PR fixs this issue. Can this PR be merged?

[justaugustus]

Thanks!
/lgtm
/approve

[k8s-ci-robot]

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