Guard yet another /bin/sh `for in` loop against empty vars
Guard yet another /bin/sh `for in` loop against empty vars

This is a follow-up to df6794035f1e4397d89896f329525e5368b7d1cc
which missed `INSTALL_BINS`

ezyang/ghc,ezyang/ghc,shlevy/ghc,shlevy/ghc,ezyang/ghc,ezyang/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,ezyang/ghc,shlevy/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,ezyang/ghc,shlevy/ghc,ezyang/ghc,shlevy/ghc,shlevy/ghc,shlevy/ghc