[CI] Use debug build for x86_64 macOS jobs

[giordano]

Try to debug #867.

[giordano]

Now I need to go, but if someone wanted to have a look at this I set up debug with tmate. Once you log in, you can run

julia --color=yes --code-coverage=user --depwarn=yes --check-bounds=yes --warn-overwrite=yes --inline=yes --startup-file=no --project=test -e 'using Pkg; Pkg.instantiate()'
lldb -- julia --color=yes --code-coverage=user --depwarn=yes --check-bounds=yes --warn-overwrite=yes --inline=yes --startup-file=no --project=test test/integration/special_functions.jl

and you should be good to go. It doesn't always crash at the same point, occasionally you may have to run the test script a few times to trigger the crash.