PSA: CI compile flags edition

[raof at ubuntu.com]

Hey all,

Pursuant to Alan's comment¹ and subsequent follow-ons, in the 
interests of rapid CI turnaround Jenkins now does regular CI runs with 
DEB_BUILD_OPTIONS=noopt. This currently means it does an -O0 build, and 
will disable LTO when that lands.

These changes don't apply to the pre-autolanding builds; they will 
continue to be run with the standard Debian optimisations (-O2) and 
with LTO (again, once that lands).

For those playing at home, the relevant parameter is 
$run_landing_checks on build-2-binpkg-mir if you've got any other 
checks that should be enabled prior to landing but not every CI run.

¹: 
https://code.launchpad.net/~raof/mir/fix-and-enable-lto/+merge/282133/comments/733096