tests(devtools): use is_debug = true for local builds

[adamraine]

According to simon (in chat):

Release build do some additional bundling with rollup and minification with terser and are the slowest.

Let's use debug builds for local debugging. In theory we could also do debug builds for CI, but I would prefer to play it safe and verify everything we do is compatible with release builds in CI.

Additionally, doesn't look like devtools_dcheck_always_on does anything anymore. I couldn't find any usages of this build arg and all the dcheck stuff from this CL is gone.