introduce customizable build scripts for FreeBSD CI

[iblislin]

See also:

• move Pkg to stdlib #25705 (comment)
• move Pkg to stdlib #25705 (comment)

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cc @KristofferC

[ararslan]

This is pretty cool. I don't know that having it as separate files is ideal though.

I wonder whether it would be possible to format it as a YAML file, consistent with the other CI systems. (Not that YAML is an ideal format by any stretch of the imagination, but for some reason CI systems love it...)

[iblislin]

The motivation of separating scripts is that Buildbot can provide several stages in a build report, thus it improves the readability and eases to search.
As you can see, i.e. https://freebsdci.julialang.org/#/builders/1/builds/7001, there are 8 stages at the moment.

About the YAML, Buildbot doesn't have any support by default, maybe it need some hacking.

[iblislin]

I can merge them into single file, just make it versatile.

[iblislin]

merged via #25705