Add flag to preserve build/compilation directories

When a build fails it can be necessarily to look at the build directory after it failed. Debugging often requires inspecting logs and other files that may have been generated during the build.

Related to: https://github.com/asdf-vm/asdf/issues/74

The issue names no files or tests; start by tracing the build and compilation cleanup path in asdf and inspect what happens after a failed build. Define the flag's retention behavior, then verify that a failed build leaves its logs and generated files available for inspection.

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