Fix platform tags when cross-compiling universal2

[tuxu]

When cross-compiling for universal2-apple-darwin, the platform tags of the resulting wheel were not properly derived. This is because with forcing target_triple to None as before, it was not detected that we are in fact cross-compiling for a different target and thus the wheels were wrongly tagged for the host platform.

To make the inference work, set target_triple to a valid triple instead.

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[tuxu]

For reference, the behavior before and after this change.

maturin 1.7.0:

root@ce3c3013b66d:/tmp/hello# uname -a
Linux ce3c3013b66d 6.6.26-linuxkit #1 SMP Sat Apr 27 04:13:19 UTC 2024 aarch64 aarch64 aarch64 GNU/Linux

root@ce3c3013b66d:/tmp/hello# maturin build --release --target universal2-apple-darwin -i python3.9                  
🔗 Found pyo3 bindings with abi3 support for Python ≥ 3.9
🐍 Not using a specific python interpreter
📡 Using build options features from pyproject.toml
💻 Using `MACOSX_DEPLOYMENT_TARGET=11.0` for aarch64-apple-darwin by default
    Finished `release` profile [optimized] target(s) in 0.04s
💻 Using `MACOSX_DEPLOYMENT_TARGET=10.12` for x86_64-apple-darwin by default
    Finished `release` profile [optimized] target(s) in 0.01s
💥 maturin failed
  Caused by: Error checking for manylinux/musllinux compliance
  Caused by: Goblin failed to parse the elf file
  Caused by: Invalid magic number: 0x2000000bebafeca

With --skip-auditwheel:

[...]
📦 Built wheel for abi3 Python ≥ 3.9 to /tmp/hello/target/wheels/hello-0.1.0-cp39-abi3-linux_aarch64.whl

With the changes of this PR:

[...]
📦 Built wheel for abi3 Python ≥ 3.9 to /tmp/hello/target/wheels/hello-0.1.0-cp39-abi3-macosx_10_12_x86_64.macosx_11_0_arm64.macosx_10_12_universal2.whl

[messense]

Thanks!