Actually install pip3 so we can install aptly-ctl.

Change-Id: I3ffdca24fc2c4dd79ca6a0cb05f56f5f03d9d6bf
diff --git a/vars/functions.groovy b/vars/functions.groovy
index 4bd20b8..b5cc6bb 100644
--- a/vars/functions.groovy
+++ b/vars/functions.groovy
@@ -3,7 +3,7 @@
 def initSourceTree(boardName, needsNative = false) {
     sh """
        apt-get update
-       apt-get -y install make python curl git qemu-user-static sudo python3
+       apt-get -y install make python curl git qemu-user-static sudo python3 python3-pip
        curl https://storage.googleapis.com/git-repo-downloads/repo > /usr/local/bin/repo
        chmod a+x /usr/local/bin/repo