Fix macOS release build to use arm64 native architecture

macos-15 runner is arm64 but GOARCH was set to amd64, causing gobump
to be cross-compiled and fail to execute, resulting in missing version
in release zip filenames.

Fixes #49

Author: mattn

.github/workflows/release.yml

@@ -38,7 +38,7 @@ jobs:
 
   build_for_macos:
     name: Build for MacOS
-    runs-on: macos-15 # Use macos-15 to build darwin/amd64. macos-latest is arm64.
+    runs-on: macos-latest
     steps:
     - name: Install build dependencies
       run: brew install coreutils
@@ -50,11 +50,11 @@ jobs:
       uses: actions/setup-go@v4
       with:
         go-version: ${{ env.GO_VERSION }}
-    - name: Build amd64
+    - name: Build arm64
       env:
         CGO_ENABLED: 1
         GOOS: darwin
-        GOARCH: amd64
+        GOARCH: arm64
       run: make release
     - name: Archive artifacts
       uses: actions/upload-artifact@v4