[CI] Feature request: when uploading a rootfs image, don't allow it to override an existing upload (unless the user provides some command-line flag)

[DilumAluthge]

No description provided.

[DilumAluthge]

I'm envisioning something like this.

If the user does this:

julia llvm-passes.jl v1

Then the following happens:

• If the v1 release does not exist, then the v1 is created, and the llvm-passes.tar.gz asset is uploaded to the v1 release.
• If the v1 release exists, but the v1 release does not already have an asset by the name of llvm-passes.tar.gz, then the llvm-passes.tar.gz asset is uploaded to the v1 release.
• If the v1 release exists, and the v1 release already has an asset by the name of llvm-passes.tar.gz, then we throw an error and we do not overwrite the old llvm-passes.tar.gz asset.

In contrast, if the user does this:

julia llvm-passes.jl v1 --overwrite

Then the following happens:

• If the v1 release does not exist, then the v1 is created, and the llvm-passes.tar.gz asset is uploaded to the v1 release.
• If the v1 release exists, but the v1 release does not already have an asset by the name of llvm-passes.tar.gz, then the llvm-passes.tar.gz asset is uploaded to the v1 release.
• If the v1 release exists, and the v1 release already has an asset by the name of llvm-passes.tar.gz, then we overwrite the old llvm-passes.tar.gz asset with the new llvm-passes.tar.gz asset.