doc: Get stdlib dir from make system rather than executable

[Keno]

The make.jl docsystem driver was using Sys.STDLIB as the canonical list and location of stdlibs. When make.jl is invoked as part of the make system (e.g. make -C doc), I think it makes more sense to use make's idea of where the stdlibs are. Of course, if you're using a just-built julia to create the docs, these two notions are identical. However, the docsystem build supports (explicitly via the JULIA_EXECUTABLE option), being built with an external julia executable. In this case, we should still use the stdlib source from in tree (as we do with the ordinary base sources). A primary motivation is to let AI agents run the doctests before they submit PRs without having to do a whole julia build.

[fredrikekre]

On Windows CI:

ERROR: LoadError: IOError: readdir("C:\\workdir\\usr\\share\\julia\\stdlib\\vcut"): no such file or directory (ENOENT)
Stacktrace:
[1] _uv_error(prefix::String, c::Int32)
@ Base .\libuv.jl:107
[2] uv_error
@ .\libuv.jl:106 [inlined]
[3] _readdir(dir::SubString{String}; return_objects::Bool, join::Bool, sort::Bool)
@ Base.Filesystem .\file.jl:1075
[4] _readdir
@ .\file.jl:1068 [inlined]
[5] readdir
@ .\file.jl:997 [inlined]
[6] (::var"#8#9")()
@ Main C:\workdir\doc\make.jl:54
[7] cd(f::var"#8#9", dir::String)
@ Base.Filesystem .\file.jl:101
[8] top-level scope
@ C:\workdir\doc\make.jl:51
[9] include(mod::Module, _path::String)
@ Base .\Base.jl:309
[10] exec_options(opts::Base.JLOptions)
@ Base .\client.jl:331
[11] _start()
@ Base .\client.jl:564
in expression starting at C:\workdir\doc\make.jl:51
make[3]: *** [Makefile:57: html] Error 1
make[3]: Leaving directory '/c/workdir/doc'
make[2]: *** [Makefile:125: docs] Error 2
make[1]: *** [/c/workdir/Makefile:66: /c/workdir/doc/_build/html/en/index.html] Error 2
make: *** [Makefile:584: binary-dist] Error 2

[Keno]

Sorry. #58536.