clonebundles: stop shell quoting `HGCB_BUNDLE_BASENAME` environment variable This causes problems in `test-clonebundles-autogen.t` on Windows, because the quoted path ends up being passed to the `cp` command, which fails, because quote characters are not a legal part of a file name. I don't see any quoting in environment variables on either MSYS or WSL, even with weird ones that appear to have escape sequences like `PS1=\[\033]0;$MSYSTEM:\w\007` (in MSYS). The quoting was added back in 5ae30ff79c76, and as shown here, was causing problems even on posix when a quote was slipped into the path. (The other obvious problem is that the command is spun up shell style, which invokes `cmd.exe`, which doesn't know about `$foo` style variables. That will be addressed next, but that change didn't work without this too.)