Set deployment target for macOS binaries

[avinassh]

fixes #85

Without this patch, we will keep seeing warnings when go-libsql is run on a latest macOS machine than the libs were compiled on.

I tested this patch on my repo:

1. Before: https://github.com/avinassh/go-libsql/actions/runs/20717982667/job/59474230910 you can see the warnings in "Verify Darwin ARM64 build" step:

ld: warning: object file (/Users/runner/work/go-libsql/go-libsql/lib/darwin_arm64/libsql_experimental.a(fad98b632b8ce3cc-curve25519.o)) was built for newer macOS version (15.5) than being linked (15.0)
ld: warning: object file (/Users/runner/work/go-libsql/go-libsql/lib/darwin_arm64/libsql_experimental.a(ca4b6ef5433f5aeb-aes_nohw.o)) was built for newer macOS version (15.5) than being linked (15.0)
ld: warning: object file (/Users/runner/work/go-libsql/go-libsql/lib/darwin_arm64/libsql_experimental.a(ca8bd8684bb569fa-montgomery.o)) was built for newer macOS version (15.5) than being linked (15.0)

2. After: https://github.com/avinassh/go-libsql/actions/runs/20717997011/job/59474229277

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Thank you @ThomasGormley for the quick solution as well