ci: switch debian:buster apt debian repository URL to snapshot URL

As buster reached its EOL, the official debian repo URL doesn't
host anymore buster packages info. For this reason, change the URLs
to point to the `20250630T203427Z` snapshot, which still contains
them.

Signed-off-by: Leonardo Di Giovanna <[email protected]>

Author: ekoops

.github/workflows/ci.yml

@@ -38,6 +38,8 @@ jobs:
     steps:
       - name: Install deps ⛓️
         run: |
+          # Use 20250630T203427Z debian apt snapshot as it still contains support for buster.
+          printf "deb http://snapshot.debian.org/archive/debian/20250630T203427Z buster main\ndeb http://snapshot.debian.org/archive/debian-security/20250630T203427Z buster/updates main\ndeb http://snapshot.debian.org/archive/debian/20250630T203427Z buster-updates main" > /etc/apt/sources.list
           apt update && apt install -y --no-install-recommends curl ca-certificates build-essential git clang llvm pkg-config autoconf automake libtool libelf-dev wget libc-ares-dev libcurl4-openssl-dev libssl-dev libtbb-dev libjq-dev libjsoncpp-dev libgrpc++-dev protobuf-compiler-grpc libgtest-dev libprotobuf-dev linux-headers-${{ matrix.arch }}
 
       - name: Install a recent version of CMake ⛓️