[bldr-build] Create working directory earlier in `_build_environment()`.
[bldr-build] Create working directory earlier in `_build_environment()`.

Similar to #161, except this pulls the sane-default directory creation
of the working directory (i.e. `$BLDR_SRC_CACHE/$pkg_dirname`) earlier
into the `_build_environment()` phase. This ensures that no matter what,
a working directory exists, and will exist for the `_fix_libtool()`
phase to work without requiring a directory check.

The target directory for `_fix_libtool()` has also been updated to only
check under the Plan's working directory. The behavior previous to this
change was a little...aggressive. Worked, but aggressive ;)

be-plans/be,be-plans/be,be-plans/be,be-plans/be