new(pkg,cmd): refactored builder script logic.

[FedeDP]

What type of PR is this?

/kind cleanup /kind feature

Any specific area of the project related to this PR?

/area cmd /area pkg

What this PR does / why we need it:

Builder script has been split in 3 different scripts:

• download libs
• download headers
• build

This way, we can reuse download libs script among all of them. Moreover, it is useful to have a download headers script that is invokeable by itself, because it has the logic to download and extract headers for a given distro.

Finally, fixed a couple of things with local builder:

• redirect stderr to stdout so that we catch errors too while building
• pre initialize envMap to an empty map, instead of nil
• allow to automatically download kernel headers

Basically, what we want to achieve here is to allow falcoctl driver loader to be able to fetch KernelURLs using driverkit libraries, and then extract kernel urls using the newly provided templated bash scripts (note: this is a distro specific knowledge, we cannot just "untar" the headers), and finally pass the KERNELDIR env variable back to local build processor. In the end, this approach will enable us to drop the kernel headers dependency for Falco installations on distros supported by driverkit, at least when the headers are actually found by driverkit itself.

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Note that ~400Locs come from new _kernel.sh templates licenses.

Does this PR introduce a user-facing change?:

NONE

[FedeDP]

Here is the output for falcoctl leveraging the new kernel downloader script: https://github.com/falcosecurity/falcoctl/pull/476#issuecomment-1996780861

[FedeDP]

/cc @EXONER4TED

[poiana]

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