Capitalize OS name in release artifacts

[zeke]

This is a partial fix for #1465

This PR updates the Goreleaser config to capitalize OS names like Darwin and Linux so they match the output of uname -s, which we've long used for the installation instructions in the README.

@caarlos0 does this look right? I did a dry run locally and I see this:

$ goreleaser release --snapshot --skip=publish --clean              
...
• archives
  • skip archiving                                 binary=cog name=cog_Darwin_arm64
  • skip archiving                                 binary=cog name=cog_Darwin_x86_64
  • skip archiving                                 binary=cog name=cog_Linux_x86_64
  • skip archiving                                 binary=cog name=cog_Linux_arm64

[zeke]

Thanks @caarlos0 🙏🏼