extra/in_parallel.sh: call the right python version

We want to use the python installed as dependency to use an updated
version of the requests module (>=v2.4.2).

This regression is caused by
weaveworks/build-tools#73

extras/in_parallel.sh

@@ -19,7 +19,7 @@ shift 1
 
 INPUTS="$*"
 SCHED_NAME=parallel-$CIRCLE_PROJECT_USERNAME-$CIRCLE_PROJECT_REPONAME-$CIRCLE_BUILD_NUM
-INPUTS=$(echo "$INPUTS" | "../tools/sched" sched "$SCHED_NAME" "$CIRCLE_NODE_TOTAL" "$CIRCLE_NODE_INDEX")
+INPUTS=$(echo "$INPUTS" | python "../tools/sched" sched "$SCHED_NAME" "$CIRCLE_NODE_TOTAL" "$CIRCLE_NODE_INDEX")
 
 echo Doing "$INPUTS"
 
@@ -28,5 +28,5 @@ for INPUT in $INPUTS; do
     "$COMMAND" "$INPUT"
     RUNTIME=$(($(date +%s) - START))
 
-    "../tools/sched" time "$INPUT" "$RUNTIME"
+    python "../tools/sched" time "$INPUT" "$RUNTIME"
 done