boost: don't modify user-config.jam in-source

[valgur]

Summary

Changes to recipe: boost/[*]

Motivation

user-config.jam, which contains the necessary build info about the toolchain and external libs, is currently being modified in-place at <source_folder>/tools/build/user-config.jam. This is a pretty bad idea given that the recipe is using no_copy_source=True and that the location of the user-config.jam file is provided via a CLI argument anyway.

Details

This PR moves the user-config.jam generation from build() to generate() and stores the file in self.generators_folder.

A few other minor cosmetic changes to the recipe as well.

/cc @AbrilRBS, since you were recently adding warnings for source modifications with no_copy_source=True.

---

• Read the contributing guidelines
• Checked that this PR is not a duplicate: list of PRs by recipe
• Tested locally with at least one configuration using a recent version of Conan

[valgur]

Here's an example of what the user-config.jam contents look like for the default config:

using zlib : 1.3.1 : <include>"/home/martin/.conan2/p/b/zlib6e767fad2584e/p/include" <search>"/home/martin/.conan2/p/b/zlib6e767fad2584e/p/lib" <name>z ;
using bzip2 : 1.0.8 : <include>"/home/martin/.conan2/p/b/bzip2196a3629ff437/p/include" <search>"/home/martin/.conan2/p/b/bzip2196a3629ff437/p/lib" <name>bz2 ;
using "clang-linux" :  :  "/usr/bin/clang++-18" : 
<compileflags>"-I/home/martin/.conan2/p/b/libbacfc60a05f6f20/p/include" <linkflags>"-L/home/martin/.conan2/p/b/libbacfc60a05f6f20/p/lib"  ;

[valgur]

Regarding

A job has not been automatically scheduled because author has too many pull requests open.

Should I close about 200 out of my 214 currently open PRs to see any kind of CI activity at all? I doubt you're keen on keeping manually triggering these.