CI: Speed up build:devel step

• CI: Separate devel and clang-asan builds

  This build pass takes long enough that it makes sense to parallelize.

• CI: Don't run scan-build

  scan-build doesn't understand __attribute__((cleanup)) (see https://bugs.llvm.org/show_bug.cgi?id=3888) so it will report increasingly many false-positive memory leaks in this codebase. It's sufficiently time-consuming that it doesn't make sense to run it as part of our usual pipeline.