make fnm-macos.zip a universal macos executable (#1310)

GitHub runners changed from Intel-based hardware to M1, which is good, but also a breaking change in the installation script. So now we create a (fatter) executable that can run on both M1 and x64. We might have fnm-macos-arm.zip later on, but homebrew is probably a better choice!

.changeset/curvy-onions-reflect.md

@@ -0,0 +1,5 @@
+---
+"fnm": patch
+---
+
+make github releases of macos to be a universal macos executable (both m1 and x64) as github changed the workers to be m1

.github/workflows/rust.yml

@@ -75,10 +75,16 @@ jobs:
       - uses: hecrj/setup-rust-action@v1
         with:
           rust-version: ${{env.RUST_VERSION}}
+          targets: x86_64-apple-darwin,aarch64-apple-darwin
       - uses: Swatinem/rust-cache@v2
+      - uses: mlugg/setup-zig@v1
+      - run: cargo install --locked cargo-zigbuild
       - uses: actions/checkout@v4
       - name: Build release binary
-        run: cargo build --release
+        run: |
+          cargo zigbuild --release --target=universal2-apple-darwin
+          mkdir -p target/release
+          cp target/universal2-apple-darwin/release/fnm target/release/fnm
         env:
           LZMA_API_STATIC: "true"
       - name: Strip binary from debug symbols