ci(release): add libc6-dev-arm64-cross for aarch64-gnu builds

gcc-aarch64-linux-gnu ships the cross compiler and binutils only;
linking glibc binaries also requires the target libc crt/dev files
(Scrt1.o, crti.o), which live in libc6-dev-arm64-cross.

Without it, the aarch64-unknown-linux-gnu build failed with:
  ld: cannot find Scrt1.o: No such file or directory
  ld: cannot find crti.o: No such file or directory

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.gitea/workflows/release.yaml

@@ -24,7 +24,10 @@ jobs:
             apt: ""
             rustflags: ""
           - target: aarch64-unknown-linux-gnu
-            apt: "gcc-aarch64-linux-gnu"
+            # gcc-aarch64-linux-gnu ships the cross compiler/binutils
+            # but no target libc; libc6-dev-arm64-cross adds Scrt1.o,
+            # crti.o, and friends needed at link time.
+            apt: "gcc-aarch64-linux-gnu libc6-dev-arm64-cross"
             rustflags: ""
           - target: x86_64-unknown-linux-musl
             apt: "musl-tools"