* Fix accelerate launch --cpu never setting the KMP variables

#3912 gated KMP_AFFINITY and KMP_BLOCKTIME on CPU mode with:

    parse_flag_from_env(current_env["ACCELERATE_USE_CPU"], False)

`parse_flag_from_env` takes an environment variable *name*, not a value, and
does `os.environ.get(key, str(default))`. It is handed the string "True" or
"False" here, looks for an environment variable by that name, does not find
one and returns the default. The guard is therefore always False, so the two
variables are never set, including for `accelerate launch --cpu`.

Before #3912 the condition was `if current_env["ACCELERATE_USE_CPU"]:`, which
was always True because "False" is a non-empty string, so KMP_AFFINITY was set
on GPU launches too. That was the bug #3912 set out to fix; it swapped one
constant for the other.

Test the flag itself, the way the line above it already does, and drop the
import that was added only for the broken call.

* Update src/accelerate/utils/launch.py

* Update src/accelerate/utils/launch.py

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Co-authored-by: Marc Sun <57196510+SunMarc@users.noreply.github.com>