attempt to concatenate a nil value

Nobody has claimed this yet.

Using this command:

~~~
luarocks install Lua-cURL
~~~

I get this result:

~~~
Installing https://luarocks.org/lua-curl-0.3.9-1.src.rock

Error: LuaRocks 3.0.4 bug (please report at https://github.com/luarocks/luarocks/issues).
Arch.: win32-x86 (fs_use_modules = true)
[string "src/luarocks/fs/win32.lua"]:275: attempt to concatenate a nil value
stack traceback:
[string "src/luarocks/fs/win32.lua"]:275: in function 'luarocks.fs.make_temp_dir'
[string "src/luarocks/fetch.lua"]:69: in function 'luarocks.fetch.fetch_url_at_temp_dir'
[string "src/luarocks/fetch.lua"]:138: in function 'luarocks.fetch.fetch_and_unpack_rock'
[string "src/luarocks/cmd/build.lua"]:62: in function <[string "src/luarocks/cmd/build.lua"]:55>
(...tail calls...)
[string "src/luarocks/cmd/build.lua"]:155: in function 'luarocks.cmd.build.command'
(...tail calls...)
[C]: in function 'xpcall'
[string "src/luarocks/cmd.lua"]:484: in function 'luarocks.cmd.run_command'
[string "luarocks"]:35: in main chunk
[C]: in ?
~~~

Reproduce the failure with `luarocks install Lua-cURL` on win32-x86, then start at `src/luarocks/fs/win32.lua:275` and follow the calls through `src/luarocks/fetch.lua`. Check the related build and command stack shown in the report, and consider the issue done when the installation no longer raises the nil-concatenation error.

Written by the indexing model from the issue text.