Pin python version to python3 (#194)

Fixes the current broken performance CI in mmtk-core.

Author: k-sareen

.github/scripts/ci-checkout.sh

@@ -3,8 +3,8 @@ set -ex
 
 . $(dirname "$0")/common.sh
 
-OPENJDK_URL=`cargo read-manifest --manifest-path=$BINDING_PATH/mmtk/Cargo.toml | python -c 'import json,sys; print(json.load(sys.stdin)["metadata"]["openjdk"]["openjdk_repo"])'`
-OPENJDK_VERSION=`cargo read-manifest --manifest-path=$BINDING_PATH/mmtk/Cargo.toml | python -c 'import json,sys; print(json.load(sys.stdin)["metadata"]["openjdk"]["openjdk_version"])'`
+OPENJDK_URL=`cargo read-manifest --manifest-path=$BINDING_PATH/mmtk/Cargo.toml | python3 -c 'import json,sys; print(json.load(sys.stdin)["metadata"]["openjdk"]["openjdk_repo"])'`
+OPENJDK_VERSION=`cargo read-manifest --manifest-path=$BINDING_PATH/mmtk/Cargo.toml | python3 -c 'import json,sys; print(json.load(sys.stdin)["metadata"]["openjdk"]["openjdk_version"])'`
 
 rm -rf $OPENJDK_PATH
 git clone $OPENJDK_URL $OPENJDK_PATH