build: fix system tests, move to Kokoro (#372)

diff --git a/scripts/decrypt-secrets.sh b/scripts/decrypt-secrets.sh
index e02bfc1..f0ef994 100755
--- a/scripts/decrypt-secrets.sh
+++ b/scripts/decrypt-secrets.sh
@@ -20,8 +20,11 @@
 # Work from the project root.
 cd $ROOT
 
-openssl aes-256-cbc -k "$1" \
-    -in system_tests/secrets.tar.enc \
-    -out system_tests/secrets.tar -d
+gcloud kms decrypt \
+  --location=global \
+  --keyring=ci \
+  --key=kokoro-secrets \
+  --ciphertext-file=system_tests/secrets.tar.enc \
+  --plaintext-file=system_tests/secrets.tar
 tar xvf system_tests/secrets.tar
 rm system_tests/secrets.tar