ci: strip -std=c++17 from rocksdb.pc Cflags

RocksDB's install-shared writes a rocksdb.pc with both:

  -isystem third-party/gtest-1.8.1/fused-src
  -std=c++17

The previous PR fix scrubbed the bad include path but left -std=c++17.
pkg-config consumers inherit that flag via INTERFACE_COMPILE_OPTIONS,
which propagates to CMake imported targets as a compile option.

Result: Triangles' configure sets CXX_STANDARD 20, but the compile
command line ends up with '-std=c++20 ... -std=c++17' (rocksdb.pc's
flag comes last and wins). GCC reports:

  error: defaulted 'bool operator!=...' only available with
         '-std=c++20' or '-std=gnu++20'

Strip -std=c++17 from Cflags. Triangles sets its own standard via
CMake; the flag from rocksdb.pc was never useful anyway (consumers
should choose their own standard).

This bug only surfaced now because we replaced librocksdb-dev 6.11.4
with a locally-built RocksDB 8.9.1 — the system package's .pc didn't
have this -std flag, the freshly-built one does.

scripts/ci/build-rocksdb.sh

@@ -57,6 +57,8 @@ if [ -f "${PC_FILE}" ]; then
     sed -i \
         -e "s|-isystem third-party/gtest-1.8.1/fused-src|-I${INSTALL_PREFIX}/include|g" \
         -e "s|-isystem \\\${prefix}/third-party/gtest-1.8.1/fused-src|-I${INSTALL_PREFIX}/include|g" \
+        -e 's|-std=c++17 ||g' \
+        -e 's|-std=c++17$||g' \
         "${PC_FILE}"
 fi