Add apk fallback for native dotnet deps on musl/Alpine runners

dotnet-install.sh silently picks the linux-musl-x64 SDK on this runner,
which has no apt-get - only apk - so the previous apt-only fix still failed.

scripts/ci/lib.sh

@@ -65,15 +65,20 @@ ensure_dotnet_native_deps() {
   fi
 
   log "libstdc++.so.6 missing; installing native runtime deps for dotnet"
-  if command -v apt-get > /dev/null 2>&1; then
   local sudo_cmd=""
   if [[ "$(id -u)" -ne 0 ]] && command -v sudo > /dev/null 2>&1; then
     sudo_cmd="sudo"
   fi
+
+  if command -v apt-get > /dev/null 2>&1; then
     $sudo_cmd apt-get update -y
     $sudo_cmd apt-get install -y --no-install-recommends libstdc++6 libgcc-s1 libicu-dev ca-certificates
+  elif command -v apk > /dev/null 2>&1; then
+    # Alpine/musl runner - dotnet-install.sh falls back to the linux-musl-x64 SDK
+    # here, which still wants a real libstdc++/libgcc (not just gcompat).
+    $sudo_cmd apk add --no-cache libstdc++ libgcc icu-libs ca-certificates
   else
-    fail "libstdc++.so.6 missing and apt-get unavailable; install a C++ runtime manually on this runner"
+    fail "libstdc++.so.6 missing and neither apt-get nor apk is available; install a C++ runtime manually on this runner"
   fi
 }