fix clang-tidy
fix clang-tidy

error: pass by value and use std::move [modernize-pass-by-value,-warnings-as-errors]
    EnvVarScope(const std::string& name, const std::string& value) : name_(name) {
                ^~~~~~~~~~~~~~~~~~                                         ~~~~~
                std::string                                                std::move()

chainer/chainer,hvy/chainer,wkentaro/chainer,ktnyt/chainer,hvy/chainer,wkentaro/chainer,niboshi/chainer,wkentaro/chainer,chainer/chainer,ktnyt/chainer,okuta/chainer,jnishi/chainer,okuta/chainer,okuta/chainer,niboshi/chainer,jnishi/chainer,hvy/chainer,okuta/chainer,chainer/chainer,ktnyt/chainer,jnishi/chainer,tkerola/chainer,keisuke-umezawa/chainer,keisuke-umezawa/chainer,keisuke-umezawa/chainer,hvy/chainer,chainer/chainer,keisuke-umezawa/chainer,niboshi/chainer,jnishi/chainer,ktnyt/chainer,wkentaro/chainer,niboshi/chainer,pfnet/chainer