add a type assert to read on a Cmd (#48334)

(cherry picked from commit 1bff32b)

base/process.jl

@@ -447,7 +447,7 @@ function read(cmd::AbstractCmd)
  procs = open(cmd, "r", devnull)
  bytes = read(procs.out)
  success(procs) || pipeline_error(procs)
- return bytes
+ return bytes::Vector{UInt8}
 end
 
 """