Set HOMEBREW_DEBUG_INSTALL when in a shell during install -d|-i

When an "install -d formula" fails, and the user gets dropped into an
interactive shell, set the HOMEBREW_DEBUG_INSTALL env var to the name
for the formula that failed. Also set variable if the user requests an
interactive install in the first place.

Note that this may be different than the formula being installed, since
it may have been a dep that failed.

Also remove todo in utils; users can now look for HOMEBREW_DEBUG_INSTALL
in their prompt command, and adjust their prompts accordingly.

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