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
#10114 (comment).

(cherry picked from commit 0b5cf15)

Author: geekosaur

validate.sh

@@ -324,6 +324,13 @@ CABAL_TESTSUITE_BDIR="$(pwd)/$BUILDDIR/build/$ARCH/$BASEHC/cabal-testsuite-3"
 CABALNEWBUILD="${CABAL} build $JOBS -w $HC --builddir=$BUILDDIR --project-file=$PROJECTFILE"
 CABALLISTBIN="${CABAL} list-bin --builddir=$BUILDDIR --project-file=$PROJECTFILE"
 
+# This was needed in some local Windows MSYS2 environments
+# but breaks CI for Windows + GHC 9.0.2, thus it is set only on non-CI executions
+# 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 "")"
+
 # header
 #######################################################################