Rename "cert" variable to improve code clarity in pemToPrivateKey function

Description:

In the `pemToPrivateKey` function, the variable name "cert" is used to store the result of `derToPrivateKey(block.Bytes)`. This naming is confusing because:

1. The variable doesn't actually contain a certificate.
2. The function is meant to return a private key, not a certificate.

Proposed change:

Rename the "cert" variable to "privateKey" or "key" to accurately reflect its contents.

- https://github.com/hyperledger/fabric-lib-go/blob/25edd1eaf5f5b4209620bcb4f13d2c0941752820/bccsp/sw/keys.go#L261
- https://github.com/hyperledger/fabric-lib-go/blob/25edd1eaf5f5b4209620bcb4f13d2c0941752820/bccsp/sw/keys.go#L453

Current code:

```go
cert, err := derToPrivateKey(block.Bytes)
if err != nil {
return nil, err
}
return cert, err
```

Suggested code:
```go
key, err := derToPrivateKey(block.Bytes)
if err != nil {
return nil, err
}
return key, err
```

Impact:

This change does not affect the functionality of the code. It's purely a readability improvement that will enhance developer experience and reduce potential confusion when maintaining or reviewing this code.