ci: Tag aarch64 builds for aarch64 rather than arm64

Our aarch64 runners are tagged with both the aarch64 and the arm64 tag,
let's use the aarch64 tag to be consistent with the architecture naming
in APK.

Part-of: <https://gitlab.postmarketos.org/postmarketOS/pmaports/-/merge_requests/8539>

.ci/lib/generate_build_jobs.py

@@ -211,7 +211,7 @@ if __name__ == "__main__":
             archtag={
                 Arch.x86_64: "shared",
                 Arch.x86: "shared",
-                Arch.aarch64: "arm64",
+                Arch.aarch64: "aarch64",
                 Arch.armv7: "qemu",
                 Arch.armhf: "qemu",
                 Arch.riscv64: "qemu",