cmake/boost: load context Jamfile before passing context-impl to b2

With WITH_ASAN, b2 runs as `b2 context-impl=ucontext headers stage` for the
build step and `b2 context-impl=ucontext install` for the install step. The
`context-impl` feature is declared in libs/context/build/Jamfile.v2, which
neither the headers/stage nor the install targets load, so b2 aborts with:

    error: unknown feature "<context-impl>"

Name the context project as a target in both commands so its Jamfile (and the
feature) loads before the build request is expanded.

This works around https://github.com/boostorg/context/issues/297, fixed
upstream in
https://github.com/boostorg/context/commit/12ac945158ae3c2373ec0c888899373218aa209f
and first released in Boost 1.88; drop it once the bundled Boost is bumped to
1.88 or newer.

Signed-off-by: Sun Yuechi <sunyuechi@iscas.ac.cn>