make dbg config default build

[barter-simsum]

very simple change, but let's wait for #469 to merge.

a non-optimized debug build should be the default since it's the one devs
specify most often. Release/fully optimized builds should be a special case.

i.e.
bazel build :urbit ->
bazel build --compilation_mode=opt :urbit

bazel build --compilation_mode=dbg :urbit ->
bazel build :urbit

[joemfb]

i disagree, more people build urbit than debug with it, and the performance impact is pretty drastic. If you know how to debug, you're capable of deoptimizing ... ;)

[barter-simsum]

you're right, I need to attend my runtime unconscious bias training. I guess a
lot of people building from source are just doing it to run hoon.