add more pipeline/cli config vars

[Gankra]

Here's various things we mess with which we might want to expose globally (applied to all tasks) and locally (scoped to a Dep).

• meta flags
  • build dir (conflated with install dir, cf cleanup build caching #4 )
  • "skip rebuilding, I want to ensure the code_id is static"
• cargo flags
  • just let them set CARGOFLAGS?
  • --release vs --debug
• rustc flags
  • just let them set RUSTFLAGS?
  • -Csplit-debuginfo=packed
  • -Cdebuginfo=2
• minidump-stackwalk flags
  • --symbols-path
  • --symbols-url (we currently don't use mozilla's servers, which makes --debugger produce way better output)
  • --brief
  • --pretty

Also it's possible we should look into https://crates.io/crates/config to streamline this