Use path.package in package resolution tests

[t-kalinowski]

Summary

• Replace .libPaths()[[1]] heuristics in the test fixtures with direct path.package() checks.
• Drop the temporary IR_EXPECT_CACHE_DIR plumbing from the CLI tests.
• Keep the assertions focused on the package path actually reported by R, which makes the tests simpler and less brittle.

Internal changes

• Updated the script, Quarto, and reticulate fixtures to compare path.package() output against the resolved library path.
• Simplified the inline CLI expression test to use the same direct package-path check.
• Removed the extra environment variable setup that was only supporting the old heuristic.