SSBC-EM2.3-CACHE2: Step-wise caching

Created by: mrnugget

Part of https://github.com/sourcegraph/sourcegraph/issues/26142

• Extract implementation of StepCacheKey from src-cli to sourcegraph/lib 2d
• When transforming job: check database for step cache entries, add each cache hit as a VirtualMachineFile to job 1d
• src-cli: change src batch exec to read these files (could be as easy as setting -cache-dir) by changing the custom implemention of ExecutionCache 1d
• In workspace resolver: check database for step cache entries (try to not duplicate code), mark each step as "skipped" and "cached result found" 0.5d
• UI: Correctly show skipped/cached steps 0.5d