Don't use `self.date` unconditionally for `program_out_of_date()`

This avoids unnecessary cache invalidations for programs not affected by
the stage0 version (which is everything except the stage0 compiler
itself).

The redundant invalidations weren't noticed until now because they only
showed up on stage0 bumps, at which point people are used to rebuilding
everything anyway. I noticed it because I wasn't adding `self.date` to
the stamp file (because I didn't realize it was necessary). Rather than
adding self.date I thought it was better to remove it from the cache
key.

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