Fix builds for MacOS amd64 platform

[kurtismullins]

Description of Changes

The builds for Intel Mac are actually targeting Apple Silicon (arm64). I believe this is due to a change in Github's MacOS runner's newer generations.

The change here explicitly targets the Intel architecture.

API and ABI breaking changes

n/a

Expected complexity level and risk

1 - If it doesn't work, CI should fail and we'll know about it before any harm is done.

Testing

• Run it and verify that it produces a binary for the correct architeceture now

Build Log from this branch: https://github.com/clockworklabs/SpacetimeDB/actions/runs/10423008363

Local output testing that the files were built for the correct platforms:

kurtis@mbp fix-mac-on-intel-builds % ls
spacetime.darwin-amd64.tar.gz spacetime.darwin-arm64.tar.gz
kurtis@mbp fix-mac-on-intel-builds % tar xzvf spacetime.darwin-amd64.tar.gz
x spacetime
kurtis@mbp fix-mac-on-intel-builds % ls
spacetime                     spacetime.darwin-amd64.tar.gz spacetime.darwin-arm64.tar.gz
kurtis@mbp fix-mac-on-intel-builds % file spacetime
spacetime: Mach-O 64-bit executable x86_64
kurtis@mbp fix-mac-on-intel-builds % mv spacetime spacetime_x86_64
kurtis@mbp fix-mac-on-intel-builds % tar xzvf spacetime.darwin-arm64.tar.gz
x spacetime
kurtis@mbp fix-mac-on-intel-builds % file spacetime
spacetime: Mach-O 64-bit executable arm64
kurtis@mbp fix-mac-on-intel-builds % mv spacetime spacetime_arm64
kurtis@mbp fix-mac-on-intel-builds % ls
spacetime.darwin-amd64.tar.gz spacetime.darwin-arm64.tar.gz spacetime_arm64               spacetime_x86_64
kurtis@mbp fix-mac-on-intel-builds % file spacetime_x86_64 spacetime_arm64
spacetime_x86_64: Mach-O 64-bit executable x86_64
spacetime_arm64:  Mach-O 64-bit executable arm64