Make the Windows tempdir default to something under the CI directory and explicitly create it.

Turns out, mktemp doesn't like being passed a non-existent directory.

--
MOS_MIGRATED_REVID=115454169
diff --git a/scripts/ci/windows/compile_windows.sh b/scripts/ci/windows/compile_windows.sh
index ff1ea40..ae94928 100755
--- a/scripts/ci/windows/compile_windows.sh
+++ b/scripts/ci/windows/compile_windows.sh
@@ -34,7 +34,8 @@
 
 # These variables are temporarily needed for Bazel
 export BAZEL_SH="c:/tools/msys64/usr/bin/bash.exe"
-export TMPDIR="c:/temp"
+export TMPDIR=${TMPDIR:-c:/bazel_ci/temp}
+mkdir -p "${TMPDIR}"  # mkdir does work with a path starting with 'c:/', wow
 
 # Even though there are no quotes around $* in the .bat file, arguments
 # containing spaces seem to be passed properly.