Don't clobber existing CMAKE_CXX_FLAGS_RELEASE

[swolchok]

We were likely losing -DNDEBUG. Preserve our current intent to clobber the -O flag in particular, though.

[swolchok]

Stack from ghstack (oldest at bottom):

• Don't default to -fno-exceptions -fno-rtti #7746
• Don't repeat default for CMAKE_CXX_FLAGS_DEBUG #7743
• -> Don't clobber existing CMAKE_CXX_FLAGS_RELEASE #7742

[pytorch-bot]

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❌ 1 New Failure

As of commit 37114d8 with merge base cd0e584 ():

NEW FAILURE - The following job has failed:

• pull / test-phi-3-mini-runner-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 018c3c993574ac4d9c83f31d984503b3dfc194f9c94bae809f16dd1f3d399f09 /exec failed with exit code 2

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