On Linux, `tempname` ignores the `parent` argument if the `TMPDIR` environment variable is set

[DilumAluthge]

The docstring for tempname says:

If a parent directory argument is given, the temporary path will be in that directory instead.

However, this is not always true. On Linux, if the TMPDIR environment variable is set, then the temporary file paths generated by tempname will always be in the TMPDIR directory, even if the parent argument is passed to the tempname function.

Is this a bug, or is this expected behavior?

Demonstration:

julia> delete!(ENV, "TMPDIR");

julia> d1 = mktempdir()
"/tmp/jl_oWiuuo"

julia> t1 = tempname(d1)
"/tmp/jl_oWiuuo/jl_AOblx7"

julia> dirname(t1)
"/tmp/jl_oWiuuo"

julia> dirname(t1) == d1
true

julia> ENV["TMPDIR"] = joinpath(mktempdir(), "my_custom_tmpdir")
"/tmp/jl_hR2Ra5/my_custom_tmpdir"

julia> mkpath(ENV["TMPDIR"])
"/tmp/jl_hR2Ra5/my_custom_tmpdir"

julia> d2 = mktempdir()
"/tmp/jl_hR2Ra5/my_custom_tmpdir/jl_tuYj5d"

julia> t2 = tempname(d2)
"/tmp/jl_hR2Ra5/my_custom_tmpdir/jl_qhfofs"

julia> dirname(t2)
"/tmp/jl_hR2Ra5/my_custom_tmpdir"

julia> dirname(t2) == d2
false

julia> delete!(ENV, "TMPDIR");

julia> d3 = mktempdir()
"/tmp/jl_9qlOId"

julia> t3 = tempname(d3)
"/tmp/jl_9qlOId/jl_ewuK8b"

julia> dirname(t3)
"/tmp/jl_9qlOId"

julia> dirname(t3) == d3
true

[KristofferC]

Dup of #35785 (and #38873).