build(base58): declare the base58 lint tasks' dependency on generated fixtures

[bmc08gt]

code/cash is red at HEAD, so every open PR is currently failing on a build-config error unrelated to its own changes.

#1289 introduced a GenerateTestFixtures task in :libs:encryption:base58, wired into the build via commonTest { kotlin.srcDir(generateTestFixtures) }. That form carries the task dependency to the Kotlin compile tasks only — AGP's generateAndroidHostTestLintModel and lintAnalyzeAndroidHostTest read the same source directories straight off disk, so Gradle fails on an undeclared dependency on build/generated/testFixtures once both land in one task graph. That is exactly what CI's flipcashTestDebug :apps:flipcash:app:lintDebug does:

A problem was found with the configuration of task
':libs:encryption:base58:generateAndroidHostTestLintModel' (type 'LintModelWriterTask').
Property has implicit dependency
Task ':libs:encryption:base58:generateAndroidHostTestLintModel' uses this output of task
':libs:encryption:base58:generateTestFixtures' without declaring an explicit or implicit dependency.

This is the same wiring #1288 added to :libs:codes:kikcode a minute earlier. #1289's own CI was already failing on this when it merged.

Follow-up

The GenerateTestFixtures task classes in base58 and kikcode are now byte-identical apart from the generated package name. That copy-paste is why this broke twice — extracting it into a build-logic convention plugin that wires the lint dependency automatically would stop a third recurrence.