Log Message:
sysutils/bkt: update to 0.6.0

What's Changed
 - Support environment variables BKT_TTL, BKT_SCOPE, and BKT_CACHE_DIR as
   alternatives for flags --ttl, --scope, and --cache-dir, respectively (#15)
 - Added support for keying the cache off one or more files' last-modified time
   via the --modtime flag (#10).
 - --discard_failures is included in the cache key, meaning bkt -- foo and bkt
   --discard_failures -- foo will be cached separately.
 - Added CI test coverage of feature="debug" behavior.
 - Fixed flaky tests that relied unnecessarily on how file modtimes change (#14)

API Changes
 - Bkt::discard_failures() is now set on CommandDesc, and affects the cache key.
 - Added a CommandState type, making CommandDesc safe to persist and pass
   around. Environment details like the working directory and environment
   variables are captured when the CommandState instance is constructed.
 - Bkt::retrieve and Bkt::refresh now return more metadata about the cache state
   (e.g. whether there was a cache hit or miss).
 - Upgraded to Clap v4 (#25).