Fix binary-dist target with xz/gzip
Fix binary-dist target with xz/gzip

This was harmless but annoying: we forgot to take the compression
extention into account when copying the binary dist out of bindistprep

Signed-off-by: Austin Seipp <7ea35d812706d9213868749011af1ed4fa2f6aa0@well-typed.com>
(cherry picked from commit 025a66e7fc416ab9ee1df56ad26dfe6232d32900)

lukexi/ghc-7.8-arm64,hferreiro/replay,lukexi/ghc-7.8-arm64,lukexi/ghc-7.8-arm64,hferreiro/replay,hferreiro/replay,lukexi/ghc-7.8-arm64,hferreiro/replay,hferreiro/replay,lukexi/ghc-7.8-arm64,hferreiro/replay,hferreiro/replay