feat(ci): add flawfinder action

[ZyanKLee]

Same as with the other PR from earlier: take your time, this won't run away.

....

Here is another idea for regularly scanning the C code for probable issues: flawfinder

[...] flawfinder, a simple program that examines C/C++ source code and reports possible security weaknesses (“flaws”) sorted by risk level. It’s very useful for quickly finding and removing at least some potential security problems before a program is widely released to the public.

Scanning results with be shown in the pullrequest checks section:

as well as the security tab of the project:

So they will provide a good overview on possible improvements.

Though not all the findings have to be valid issues. Some may be conscious decisions - for example those complaints, that something can be accesses from outside of the C code (I guess at least some of that is needed to access those with the Go part of NoiseTorch?)