Fix release CI (and add sccache)

[ribbanya]

• Before
• After

sccache saves a little bit of time, it's hard to say how much. On ubuntu-latest it's like 3 minutes saved, but on Windows it's more marginal. 🤷

contents: write was necessary to avoid 403 errors on my fork.

[ribbanya]

Actually this will release the GUI as objdiff-gui and not objdiff. I can fix that, just let me know.

[encounter]

Yeah, I’d like to keep the binary name objdiff instead of -gui because of the auto updater.

[ribbanya]

Fixed.