Simplify artifact names: -macos-setup.pkg, -linux-setup.AppImage

Arch markers dropped except win64: macOS builds are arm64-only and we
ship a single Linux flavor.

scripts/guibuild.py

@@ -64,19 +64,13 @@ _VPK_STAGING = _build_cache_dir() / f"vpk-{_VPK_VERSION}"
 
 
 def _platform_zip_suffix() -> str:
-    machine = platform.machine().lower()
-    arch = {
-        "x86_64": "x64",
-        "amd64": "x64",
-        "arm64": "arm64",
-        "aarch64": "arm64",
-    }.get(machine, machine or "unknown")
-
+    # Only Windows keeps an arch marker (win64); macOS is arm64-only and we
+    # build just one Linux flavor, so -macos / -linux suffice.
     if sys.platform == "win32":
         return "win64"
     if sys.platform == "darwin":
-        return f"macos-{arch}"
-    return f"linux-{arch}"
+        return "macos"
+    return "linux"
 
 
 def fetch_ffmpeg() -> Path: