Distributed: fix syntax check of environment variable names (#38577)

- previous code accidentally checked `ENV` instead of `env`

- now separate `env` checks for POSIX shell and `cmd.exe`, which have
  quite different format constraints for their environment variable names

stdlib/Distributed/src/managers.jl

@@ -258,9 +258,6 @@ function launch_on_machine(manager::SSHManager, machine::AbstractString, cnt, pa
  env[var] = ENV[var]
  end
  end
- for var in keys(ENV)
- occursin(r"^[a-zA-Z0-9_]+$", var) || throw(ArgumentError(var))
- end
 
  # Julia process with passed in command line flag arguments
  if shell == :posix
@@ -269,6 +266,8 @@ function launch_on_machine(manager::SSHManager, machine::AbstractString, cnt, pa
  cmds = "$(shell_escape_posixly(exename)) $(shell_escape_posixly(exeflags))"
  # set environment variables
  for (var, val) in env
+ occursin(r"^[a-zA-Z_][a-zA-Z_0-9]*\z", var) ||
+ throw(ArgumentError("invalid env key $var"))
  cmds = "export $(var)=$(shell_escape_posixly(val))\n$cmds"
  end
  # change working directory
@@ -290,6 +289,7 @@ function launch_on_machine(manager::SSHManager, machine::AbstractString, cnt, pa
  end
  # set environment variables
  for (var, val) in env
+ occursin(r"^[a-zA-Z0-9_()[\]{}\$\\/#',;\.@!?*+-]+\z", var) || throw(ArgumentError("invalid env key $var"))
  remotecmd = "set $(var)=$(shell_escape_wincmd(val))&& $remotecmd"
  end