Fix Trixie cross-building

Debian 13 removed the `cross-gcc-dev` package as it was unmaintained
(Debian bug #1107126). This broke the existing cross build setup.

Instead, switch to using binary cross-gcc packages, which work the same
without having to manually generate the libraries. This is still fully
symmetrical.

build.yaml

@@ -30,8 +30,10 @@ debian:
   archmaps:
     amd64:
       PACKAGE_ARCH: amd64
+      GCC_ARCH: x86-64
     arm64:
       PACKAGE_ARCH: arm64
+      GCC_ARCH: aarch64
   cross-gcc:
     bullseye: '10'
     bookworm: '12'