kokoro: use Python 3.12 in Linux builds (#5730)

Bug: crbug.com/350048185

kokoro/scripts/linux/build-docker.sh

diff --git a/kokoro/scripts/linux/build-docker.sh b/kokoro/scripts/linux/build-docker.sh
index e47037d..b2c034d 100755
--- a/kokoro/scripts/linux/build-docker.sh
+++ b/kokoro/scripts/linux/build-docker.sh
@@ -27,6 +27,8 @@
 
 . /bin/using.sh # Declare the bash `using` function for configuring toolchains.
 
+using python-3.12
+
 if [ $COMPILER = "clang" ]; then
   using clang-10.0.0
 elif [ $COMPILER = "gcc" ]; then