CI: install libc6-dev-amd64-cross for x86_64 ring cross-build

The x86_64 cross build failed compiling ring's C (curve25519.c): the cross
gcc couldn't find bits/libc-header-start.h because only the compiler
(gcc-x86-64-linux-gnu) was installed, not the amd64 target libc headers.
Add libc6-dev-amd64-cross so the cross sysroot has them.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011YPrEZVBpPVoCRyJz2exkZ

.gitea/workflows/release.yaml

@@ -36,7 +36,11 @@ jobs:
           set -eux
           SUDO=""; [ "$(id -u)" -ne 0 ] && SUDO=sudo
           $SUDO apt-get update
-          $SUDO apt-get install -y --no-install-recommends gcc gcc-x86-64-linux-gnu jq curl ca-certificates
+          # gcc: native (arm64) C-compiler för ring. gcc-x86-64-linux-gnu + libc6-dev-amd64-cross:
+          # cross-compiler OCH x86_64-målets libc-headers/-libs (annars hittar cross-gcc:n
+          # inte bits/libc-header-start.h när ring:s C-kod byggs).
+          $SUDO apt-get install -y --no-install-recommends \
+            gcc gcc-x86-64-linux-gnu libc6-dev-amd64-cross jq curl ca-certificates
 
           # Rust (om det saknas på runnern).
           if ! command -v cargo >/dev/null 2>&1; then