`addenv()`, by default, should inherit from `ENV` (#39100)
The spirit of `addenv()` is to be non-destructive, however because a
`Cmd` object with an `env` member that is set to `nothing` (the default)
inherts from the current environment when it is run. This means that
the following, rather surprising behavior holds true on current master:
```
julia> ENV["FOO"] = "foo"
run(`/bin/bash -c "echo \$FOO \$BAR"`)
run(addenv(`/bin/bash -c "echo \$FOO \$BAR"`, "BAR" => "bar"))
foo
bar
```
This PR adds an `inherit` flag to `addenv()` to allow keeping this
behavior if it is actually desired (this might make sense if you are
constructing `.env` blocks over a series of `addenv()` calls and you
don't want to have to special-case your first operation as a `setenv()`
rather than an `addenv()`.
