www/bunkerweb: Fix rc scripts for non-3.11 Python versions

The rc scripts hardcoded /usr/local/bin/python3.11, which only worked
while the tree default happened to be 3.11. With the default at 3.12
none of the three Python services could start. Substitute the
interpreter through %%PYTHON_VERSION%% instead.

While here, create /var/run/bunkerweb in the scheduler precmd. The
scheduler writes its own pid file below that directory and died with
FileNotFoundError on a fresh boot because nothing created it.

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