add more pipeline/cli config vars

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

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