`revive.modifies-value-receiver` reports that `artifact.Checksum`
doesn't set the `artifact.Extra` field due to the use of a value
receiver in:
```go
func (a Artifact) Checksum(algorithm string) (string, error)
```

Golangci-lint output:

```console
$ golangci-lint run
internal/artifact/artifact.go:359:3: modifies-value-receiver: suspicious assignment to a by-value method receiver (revive)
                a.Extra = make(Extras)
                ^
1 issues:
* revive: 1
```

I believe this is a bug. If it’s not, we can remove the following lines
(added in #4707), since they have no effect on the original `Artifact`:

```go
if a.Extra == nil {
	a.Extra = make(Extras)
}
a.Extra[ExtraChecksum] = fmt.Sprintf("%s:%s", algorithm, check)
```