Enabling a vimrc setting from the current buffer

Greetings,

High level goal: Optionally have vim run the current buffer right after writing
it to disk.

A setting like this does make the post write run work, but on all buffers.

au BufWritePost * !./%

How can I make this work on just the current buffer and how can a quickly
toggle it on or off with less typing? I did try making a function, but calling
it did nothing:

if exists("b:Autorun")
au BufWritePost * !./%
endif

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