Reusable CI: Allow specifying targets, defines, link and variant for B2 builds

[Flamefire]

Previously it was possible to specify all the B2_* variables, but now we'd always build at least the 4 combinations of static/shared & debug/release

This PR brings that feature back as inputs like b2_variant for B2_VARIANT

Example run: https://github.com/boostorg/boost-ci/actions/runs/25934255926/job/76381752610

+ ./b2 libs/boost-ci/test//test_def toolset=gcc cxxstd=11,17 link=shared,static variant=debug -j5 'define=FOO="Hello world"' define=BAR=2

And Windows:

b2 --abbreviate-paths libs/boost-ci/test//test_def toolset=msvc-14.5 cxxstd=17,latest address-model=64 link=shared,static variant=debug -j5 define=FOO="Hello world" define=BAR=2

This shows correct argument parsing also on Windows. It's just a B2 bug now not allowing this, see https://cpplang.slack.com/archives/C27KZLB0X/p1778846649331049
I don't know why it stops at ...found 1 target... there, but probably due to or related to that.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (d304e0e) to head (a6168c6).

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #338   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            3         3           
  Lines           24        24           
  Branches        11        11           
=========================================
  Hits            24        24           

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[sdarwin]

There are a wide variety of CI scripts running.
Drone, appveyor, etc. The CI systems are leveraging boost-ci.
This PR modifies core functionality in enforce.sh and build.bat. Most likely, it is fine, but I don't follow all the details here. If you have a high level of confidence in the changes, then probably it is ok.

[Flamefire]

This PR modifies core functionality in enforce.sh and build.bat. Most likely, it is fine, but I don't follow all the details her

Quick summary:

build.bat

• move cd BOOST_ROOT, setting B2_TARGETS further up -> no change
• Handle new variable B2_FULL_ARGS if set -> No change unless someone happened to use that somehow

enforce.sh

• fallback if Python is not found at all instead of hard error
• Don't set cxxstd if B2_CXXSTD is not set
• handle B2_TARGET_OS (usually only set in Windows)
• move B2_DEFINES/B2_INCLUDE handling to get a single B2_ARGS (initial) array
• handle the clang-win toolset there, similar to what is in build.bat
• add B2_FLAGS at the very end, not before possible define= & include= entries. (allow overwrites)

So I'm very sure this does not cause any regressions as it is mostly refactoring and making things more consistent & robust.