Feature request: when using addenv, allow "FOOBAR" => nothing to delete the FOOBAR environment variable

[DilumAluthge]

When using the withenv function, I can pass "FOOBAR" => nothing in order to delete the FOOBAR environment variable:

julia> cmd = `$(Base.julia_cmd()) -e '@info "" haskey(ENV, "FOOBAR"); @info "" get(ENV, "FOOBAR", "not found")'`;

julia> ENV["FOOBAR"] = "hello world"
"hello world"

julia> withenv() do
           run(cmd);
       end;
┌ Info:
└   haskey(ENV, "FOOBAR") = true
┌ Info:
└   get(ENV, "FOOBAR", "not found") = "hello world"

julia> withenv("FOOBAR" => nothing) do
           run(cmd);
       end;
┌ Info:
└   haskey(ENV, "FOOBAR") = false
┌ Info:
└   get(ENV, "FOOBAR", "not found") = "not found"

However, currently, I cannot use the same syntax to delete an environment variable when using the addenv function:

julia> cmd = `$(Base.julia_cmd()) -e '@info "" haskey(ENV, "FOOBAR"); @info "" get(ENV, "FOOBAR", "not found")'`;

julia> ENV["FOOBAR"] = "hello world"
"hello world"

julia> run(addenv(cmd));
┌ Info:
└   haskey(ENV, "FOOBAR") = true
┌ Info:
└   get(ENV, "FOOBAR", "not found") = "hello world"

julia> run(addenv(cmd, "FOOBAR" => nothing));
┌ Info:
└   haskey(ENV, "FOOBAR") = true
┌ Info:
└   get(ENV, "FOOBAR", "not found") = "nothing"

I think it would be nice if run(addenv(cmd, "FOOBAR" => nothing)) deleted the FOOBAR environment variable when running the command, instead of the current behavior (in which it sets the value of the FOOBAR environment variable to a string with the contents "nothing").

[DilumAluthge]

cc: @staticfloat

[fredrikekre]

#43271