fix non-POSIX [[ ]]

Otherwise CI prints `validate.sh: 332: [[: not found` and the line
does nothing (but `validate.sh` continues to run), unless the
system shell is `ksh` / `bash` / `zsh`. This may explain
https://github.com/haskell/cabal/pull/10114#issuecomment-2178163927.

validate.sh

@@ -329,7 +329,7 @@ CABALLISTBIN="${CABAL} list-bin --builddir=$BUILDDIR --project-file=$PROJECTFILE
 # of validate.sh
 # https://github.com/haskell/cabal/issues/9571
 # https://github.com/haskell/cabal/pull/10114
-RTSOPTS="$([[ $ARCH = "x86_64-windows" && -z "$CI" ]] && echo "+RTS --io-manager=native" || echo "")"
+RTSOPTS="$([ $ARCH = "x86_64-windows" ] &&  [ -z "$CI" ] && echo "+RTS --io-manager=native" || echo "")"
 
 # header
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