Fix multiarch builds on macOS

[dharmab]

The build_macos actions workflow incorrectly tried to build both the Intel and Apple Silicon executables on the same macOS runner. At the time of the last release, this built a pair of identical x86_64 binaries because macos-latest pointed at an Intel machine. If one were to run a release today, this would build a pair of identical arm64 binaries because macos-latest now points at a modern Mac.

As a result, the Apple Silicon release of piper has been broken on new macs for at least a year.

This PR uses the older macos-13 runners for the x86_64 build and macos-latest for the arm64 build. I made a test release at https://github.com/dharmab/piper/releases/tag/2024.12.14.1-alpha2 that seems to work:

[dharmab@saber Downloads] tar xf piper_macos_x64.tar.gz
[dharmab@saber Downloads] file piper/piper
piper/piper: Mach-O 64-bit executable x86_64
[dharmab@saber Downloads] rm -rf piper
[dharmab@saber Downloads] tar xf piper_macos_aarch64.tar.gz
[dharmab@saber Downloads] file piper/piper
piper/piper: Mach-O 64-bit executable arm64

A new release will need to be tagged after this PR is merged.

Fixes #523
Fixes #480
Fixes #284