macOS CI is slow

The macOS runners on GH are notoriously overloaded.

A normal borg test run already takes long, but the tagged runs (like for a release, including testing of the binary) take even much longer, might even run into a max runtime timeout after 6h.

Guess we might need to exclude the binary from testing to avoid the timeout. Or try the `borg-dir` binary type, that might be also faster.