build: pass zig lib dir as directory instead of as string

[Jan200101]

I can't build langref on 0.14.0 or master because a relative path is passed to doctest and doctest is ran in a temporary directory.

alex pointed me to #23122, but that is a more radical solution to the problem, this PR is more modest by checking if graph.zig_lib_directory.path is set and then passing it via addDirectoryArg.

tested by running zig build docs

https://zsf.zulipchat.com/#narrow/channel/454360-compiler/topic/unable.20to.20open.20zig.20lib.20directory.20when.20building.20docs/with/513624116

[Jan200101]

I needed to add a check for absolute paths because zig_lib_directory makes no guarantee of being absolute or relative (its absolute in the CI and relative on my system, haven't checked why this is) and std.Build.path panics on absolute paths

[mlugg]

Incorrect fix; the path is either absolute or cwd-relative, both of which are correct to pass directly to the child, and neither of which is correct to pass through b.path (which is for build-root-relative paths). The correct fix was #23894.