`run` with `stdin=IOBuffer(str)` doesn't work with str a substring

[c42f]

The following pipeline works as expected:

julia> run(pipeline(`cat`, stdin=IOBuffer("Hello world\n")))
Hello world
Process(`cat`, ProcessExited(0))

But if the IOBuffer contains a substring (eg, from split) a method error occurs:

julia> run(pipeline(`cat`, stdin=IOBuffer(SubString("Hello world\n"))))
ERROR: MethodError: no method matching rawhandle(::Base.GenericIOBuffer{SubArray{UInt8,1,Array{UInt8,1},Tuple{UnitRange{Int64}},true}})
Closest candidates are:
  rawhandle(::RawFD) at cmd.jl:161
  rawhandle(::Base.DevNull) at cmd.jl:160
  rawhandle(::Base.Filesystem.File) at filesystem.jl:75

[laborg]

This works on master now.

julia> versioninfo()
Julia Version 1.8.0-DEV.1440
<snip>

julia> run(pipeline(`cat`, stdin=IOBuffer(SubString("Hello world\n"))))
Hello world
Process(`cat`, ProcessExited(0))

[fredrikekre]

Fixed and tested in #40780