src: fix compiler warning in env.cc

 ../src/env.cc:1227:22: error: lambda capture 'this' is not used [-Werror,-Wunused-lambda-capture]
  ForEachBaseObject([this](BaseObject* obj) {
                 ^~~~

(I don’t think we need to discuss fast-tracking here, but you can feel free to 👍 this either way.)

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines