Local channels improvement

[Hind-M]

As mentioned in #2853 (comment), we would like to test these local channels paths without having to create a folder in the home directory.
On another hand, the local channels only work if the conda-bld folder exists in one of the following:
home_dir, target_prefix and root_prefix.
A way to improve things, is to allow searching for any conda-bld folder which could exist anywhere within these three root directories.
A test could then use a temporary directory somewhere...
What do you think @AntoinePrv ?