buildscripts: always make sure we are using jdk8 (grpc#8800)

Author: sanjaypujare

buildscripts/kokoro/unix.sh

@@ -24,6 +24,9 @@ fi
 cd $(dirname $0)/../..
 
 # TODO(zpencer): always make sure we are using Oracle jdk8
+if [[ -f /usr/libexec/java_home ]]; then
+    JAVA_HOME=$(/usr/libexec/java_home -v"1.8.0")
+fi
 
 # ARCH is x86_64 unless otherwise specified.
 ARCH="${ARCH:-x86_64}"