[core] Implement scoped env setter

[dentiny]

When working on pipe logger (#50248), I found it necessary to test based on a scoped env, so make a formal util.

[jjyao]

@dayshah could you review?

[edoakes]

@dentiny don't see it used anywhere -- are there tests you'd update to use this?

[dentiny]

@dentiny don't see it used anywhere -- are there tests you'd update to use this?

It's used here:

ray/src/ray/util/tests/pipe_logger_test.cc

Lines 37 to 43 in be1b38f

	class PipeReadBufferSizeSetter {
	public:
	PipeReadBufferSizeSetter(size_t pipe_buffer_size) {
	setEnv(kPipeLogReadBufSizeEnv.data(), absl::StrFormat("%d", pipe_buffer_size).data());
	}
	~PipeReadBufferSizeSetter() { unsetEnv(kPipeLogReadBufSizeEnv.data()); }
	};

The PR's still pending review, not merged yet

[dayshah]

lgtm but maybe overkill to have a utility if we only have one use case for it rn?

[dayshah]

idk how this will be used but are races with setenv a concern -https://en.cppreference.com/w/cpp/utility/program/getenv?

[dentiny]

Only expect to be useful in unit tests at the moment, not a big concern for thread-safety.
Speaking of which, strerror is also not thread-safe, might be a bigger concern.

[dentiny]

lgtm but maybe overkill to have a utility if we only have one use case for it rn?

I imagine it'd be useful in unit tests, no intended use cases other than that.