Revert "tools: remove libvips from exec_environ"

The rationale in the reverted change isn't true, as libvips15 is not
only installable, but in fact perfectly installed on all exec nodes.

What is missing since August 2013 is (Coren's fixed) version of
libvips-dev that is required by the bastion hosts (dev_environ):

| Error: /Stage[main]/Toollabs::Dev_environ/Package[libvips-dev]/ensure: change from purged to latest failed: Could not update: Execution of '/usr/bin/apt-get -q -y -o DPkg::Options::=--force-confold install libvips-dev' returned 100: Reading package lists...

This reverts commit 02cbb091c3b91583f85b768718ce0933d0e9d2c3.

Bug: 52717
Change-Id: I3aa0c478b38248b5edf6f959aef1029d1205564a