github/workflows: Build unix port for docs and run workflow more often.

The unix port is needed to build the docs, due to the cpydiff tests which
run both CPython and MicroPython (unix port).  That was previously not
failing the CI because the output from MicroPython was:

    /bin/sh: 1: ../ports/unix/build-standard/micropython: not found

which doesn't match the CPython output for any of the cpydiff tests, and so
it was considered a "pass" in terms of the output differing.

Also, run the docs workflow when py/ or tests/cpydiff/ changes, because the
cpydiff results may change when the core code changes.

Signed-off-by: Damien George <damien@micropython.org>

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