test-build: fix container stdin and move lint in-container

Two bugs surfaced by the first real end-to-end run:

1. docker run lacked -i, so bash -s got no stdin and the heredoc build script
   was silently discarded (0s BUILD-FAILED, empty log). Add -i.

2. sbopkglint ran host-side, where it shells out to sudo to installpkg the
   package for inspection; Slackware has no configured sudo, so lint errored
   (lxsudo not found / password required). Move the lint into the build
   container (target only, gated on IS_TARGET), where it runs as root with the
   baked-in tool and needs no sudo. The host now just reads the LINT-CLEAN /
   LINT-FINDINGS marker from the log for its summary. Drop the dead host-side
   lint_pkg function.

Verified: feroxbuster builds and lints clean end-to-end against
sbo-testbuild:current in 72s.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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