[libcxx] Use %{temp} instead of %T (#162323)

Based on review feedback in #160026.

This makes the substitution a lot more clear now that there is no
documentation around %T.

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Co-authored-by: Louis Dionne <ldionne.2@gmail.com>
GitOrigin-RevId: 0c2913afc82a683d82f16f09442d49b1fb25ca67