test: give every integration test its own bld dir on the cargo target drive

[baszalmstra]

Description

Two issues with the integration test setup in pixi_command_dispatcher:

• The six tests that omitted with_workspace(...) on their CommandDispatcher fell back to <root>/bld for source_build_workspaces(), so every test in the binary wrote into one shared bld/ tree.
• Every per-test tempdir was created via tempfile::tempdir(), which uses %TEMP%. On Windows that's typically C:\, while the project lives on a different drive. Once test(pixi_command_dispatcher): isolate the integration cache to a per… #6009 lands and the rattler cache root also lives in a tempdir, package-cache to prefix installs would be cross-drive copies instead of hardlinks.

Fix both:

• Add a test_tempdir() helper that creates tempdirs under CARGO_TARGET_TMPDIR with a short p- prefix (Windows MAX_PATH). Route every tempfile::tempdir().unwrap() in the file through it so test scratch space sits on the cargo target drive.
• Wire the six call sites without with_workspace(...) through test_tempdir() so each test gets its own bld/ (plus meta-v0/, artifacts-v0/, legacy-source-env/).

default_cache_dirs() is left alone for #6009 to handle; once that lands, its OnceLock'd cache root will share the same drive as every test_tempdir().

Supersedes #6009

How Has This Been Tested?

cargo check --tests -p pixi_command_dispatcher --features slow_integration_tests succeeds locally and cargo fmt --check is clean. The existing integration suite exercises every touched test.

AI Disclosure

• This PR contains AI-generated content.
  • I have tested any AI-generated content in my PR.
  • I take responsibility for any AI-generated content in my PR.

Tools: Claude

Checklist:

• I have performed a self-review of my own code
• I have added sufficient tests to cover my changes.

[hunger]

LGTM