ci: add app/build_gtest.sh to build and install GoogleTest from system sources

app/build_gtest.sh

@@ -0,0 +1,22 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+# Build and install GoogleTest on Debian/Ubuntu when libgtest-dev provides sources
+# Installs static libs into /usr/local so cmake can find GTest::gtest
+if [ -d /usr/src/googletest ] || [ -d /usr/src/gtest ]; then
+  echo "Building GoogleTest from system sources"
+  if [ -d /usr/src/googletest ]; then
+    src=/usr/src/googletest
+  else
+    src=/usr/src/gtest
+  fi
+  builddir="${src}/build"
+  mkdir -p "$builddir"
+  pushd "$builddir"
+  cmake -DCMAKE_POSITION_INDEPENDENT_CODE=ON ..
+  cmake --build . -- -j"$(nproc)"
+  cmake --install . --prefix=/usr/local
+  popd
+else
+  echo "No system gtest sources found in /usr/src; skipping build."
+fi