Add support for generating scripts to run benchmarks locally.

[mattmccutchen-cci]

Work in progress, to be used as part of the oopsla2022-artifact branch (which may have additional, unrelated changes for the artifact) and later polished up and merged to main. In the meantime, anyone who wants can still manually check out the run-locally branch to run benchmarks locally if they are OK with the current limitations.

Example usage:

./generate-workflow.py local --output local.sh \
  --workflow-config main --benchmark zlib --subvariant no_expand_macros_no_alltypes \
  --3c-source-dir ~/3c --3c-build-dir ~/3c/build \
  --checkedc-benchmarks-dir ~/checkedc-benchmarks \
  --work-dir ~/benchmarks/gwl-work
./local.sh

TODO:

• Stats generation: zips are saved to a stats directory in the same format as GitHub artifacts
• Error handling: --skip-post-conversion-build is available as a stopgap