Fix use-after-free warning in repl.cc

[Mic92]

NixRepl inherits from gc which uses Boehm GC for memory management. Using std::unique_ptr with GC-managed objects causes use-after-free because unique_ptr calls delete while the GC expects to manage the memory itself.

Replace std::make_unique with raw pointer allocation to let the GC handle memory management properly.

Fixes clang-tidy warning: Use of memory after it is freed [clang-analyzer-cplusplus.NewDelete]

Isolated from: #13494 because of #13494 (comment)

Motivation

Context

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[NaN-git]

NAK. The PR does not fix any use-after-free or double-free. Instead it prevents the destructor of NixRepl from being called.

First, the life-time of the allocated objects ends when the function returns. During its life-time the unique_ptr is stored on the stack, i.e. the NixRepl instance won't be GCed. The destructor of unique_ptr calls the destructor of NixRepl.

The class gc inherited by NixRepl defines custom new and delete operators that are just wrapping GC_MALLOC and GC_FREE. Thus no destructor will be called, when the object should be GCed. Calling GC_FREE explicitly is optional, but OK, too.

Maybe the warning is caused by this line of code because the constructor of StaticEnv can throw an exception.

[Mic92]

Ok. I'll find a way to make turn the warning off for this line than.