build: always run build task in jenkins (#10689)

We always run the build task now in jenkins jobs rather than only in the
core builds. This ensures that the build task runs in PR builds even
though the build artifacts are not used for anything.

Ideally all the selenium tests would run against this build rather than
it just being something that gets ran and forgotten about, but this is a
first step.

test/scripts/jenkins_unit.sh

@@ -4,3 +4,5 @@ set -e
 source "$(dirname $0)/_jenkins_setup.sh"
 
 xvfb-run "$(npm bin)/grunt" jenkins:unit;
+
+"$(npm bin)/grunt" build;