As of December 3, the GitHub Actions version of one of the programs (not sure which one) that git-annex bundles in its DMG installer for macOS uses @loader_path in the path for one of its dynamic libraries. (The value of the @loader_path variable is the directory containing the binary file in which the path appears.) Build/OSXMkLibs.hs does not resolve the @loader_path syntax, and so this leads to ./Build/Standalone failing with:

cp: ./@loader_path/libbrotlicommon.1.dylib: No such file or directory
Standalone: library install failed
CallStack (from HasCallStack):
  error, called at ./Build/OSXMkLibs.hs:72:41 in main:Build.OSXMkLibs

I attempted to patch Build/OSXMkLibs.hs to handle this myself, but the file containing this syntax is another dynamic lib, and the code doesn't seem to record the original paths that dynamic libraries are copied from, and so the patch would be nontrivial.

done per commeents --Joey