[llvm] workflows/release-binaries: Fix digest generation on macOS (PR #176440)

[Tom Stellard via llvm-commits]

tstellar wrote:

> This fixes it on MacOS? At least locally on my Macbook, `sha256sum` works as expected.

Apparently, that isn't installed on these runners by default. 


https://github.com/llvm/llvm-project/pull/176440