CI: Fix random password generation for macOS codesigning

`sha1sum` is part of Homebrew's coreutils, but macOS ships with
`shasum` by default, which supports many variants and defaults to
SHA-1 by default.
This is essentially a cherry-pick of:
obsproject/obs-studio@1e74256

.github/actions/setup-macos-codesigning/action.yaml

@@ -75,7 +75,7 @@ runs:
 
           print -n "${MACOS_SIGNING_CERT}" | base64 --decode --output="${certificate_path}"
 
-          : "${MACOS_KEYCHAIN_PASSWORD:="$(print ${RANDOM} | sha1sum | head -c 32)"}"
+          : "${MACOS_KEYCHAIN_PASSWORD:="$(print ${RANDOM} | shasum | head -c 32)"}"
 
           print '::group::Keychain setup'
           security create-keychain -p "${MACOS_KEYCHAIN_PASSWORD}" ${keychain_path}