Make overwriting releases explicit

[janseeger]

We just ran into the issue that a release with an existing tag would just overwrite the old one without warning. I get that overwriting is useful in situations like https://github.com/softprops/action-gh-release/issues/47 where nightly should be overwritten. But can we then revert the default behaviour to non-destructive and have those who need it for nightlies explicitly set it?

[Vadorequest]

On my side, I'm having difficulties overwriting a generic v1 version, it doesn't get overridden and so far I couldn't figure out why.

So, a boolean like allow_override would be useful indeed, IMHO.