[machine] fix version timestamp for test_machine_monitor.

Change-Id: Idf96e14ec0cc8f4c3437b745e00f30012b346752
Reviewed-on: https://skia-review.googlesource.com/c/buildbot/+/434916
Reviewed-by: Joe Gregorio <jcgregorio@google.com>
diff --git a/machine/Makefile b/machine/Makefile
index 9a44d62..bc616c9 100644
--- a/machine/Makefile
+++ b/machine/Makefile
@@ -76,7 +76,7 @@
 	machineserver --local --logtostderr
 
 export HASH=$(shell git rev-parse HEAD)
-export DATETIME=$(date --utc "+%Y-%m-%dT%H:%M:%SZ")
+export DATETIME=$(shell date --utc "+%Y-%m-%dT%H:%M:%SZ")
 export GIT_STATE=$(shell ../bash/gitstate.sh)
 export VERSION=${DATETIME}-${USER}-${HASH}-${GIT_STATE}