snapcraft remote-build fails in Windows when using a \\wsl$ path

[Lucy Llewellyn]

Steps to reproduce:

1. Install snapcraft to Windows
2. Create a WSL2 instance with Ubuntu 20.04 called "Ubuntu"
3. Load the WSL2 instance and checkout a project to build
4. Navigate in PowerShell to \\wsl$\Ubuntu\path\to\project
5. Execute snapcraft remote-build in PowerShell

Output:

snapcraft remote-build is experimental and is subject to change - use with caution.
Using build ID snapcraft-home-assistant-snap-481c44ebc53271709774b19f72aff09c
The authorization page:
 (https://launchpad.net/+authorize-token?oauth_token=PbWrL5vD0XMnFBtXTSrk&allow_permission=DESKTOP_INTEGRATION)
should be opening in your browser. Use your browser to authorize
this program to access Launchpad on your behalf.
Waiting to hear from Launchpad about your decision...
All data sent to remote builders will be publicly available. Are you sure you want to continue? [Y/n]: y
Packaging sources for updater...
Sorry, an error occurred in Snapcraft:
path is on mount 'C:', start on mount '\\\\wsl$\\Ubuntu'
Traceback (most recent call last):
  File "snapcraft\cli\__main__.py", line 43, in <module>
  File "click\core.py", line 1137, in __call__
  File "click\core.py", line 1062, in main
  File "click\core.py", line 1668, in invoke
  File "click\core.py", line 1404, in invoke
  File "click\core.py", line 763, in invoke
  File "snapcraft\cli\remote.py", line 181, in remote_build
  File "snapcraft\cli\remote.py", line 219, in _start_build
  File "snapcraft\internal\remote_build\_worktree.py", line 325, in prepare_repository
  File "snapcraft\internal\remote_build\_worktree.py", line 244, in _process_part_sources
  File "snapcraft\internal\remote_build\_worktree.py", line 192, in _pull_source
  File "snapcraft\internal\sources\_local.py", line 41, in pull
  File "snapcraft\file_utils.py", line 216, in link_or_copy_tree
  File "ntpath.py", line 562, in relpath
ValueError: path is on mount 'C:', start on mount '\\\\wsl$\\Ubuntu'
We would appreciate it if you created a bug report at
https://launchpad.net/snapcraft/+filebug with the above text included.
You can find the traceback in file 'C:\\Users\\DLLEWE~1\\AppData\\Local\\Temp\\tmpi3dhep08\\trace.txt'.

Traceback:

Traceback (most recent call last):
  File "snapcraft\cli\__main__.py", line 43, in <module>
  File "click\core.py", line 1137, in __call__
  File "click\core.py", line 1062, in main
  File "click\core.py", line 1668, in invoke
  File "click\core.py", line 1404, in invoke
  File "click\core.py", line 763, in invoke
  File "snapcraft\cli\remote.py", line 181, in remote_build
  File "snapcraft\cli\remote.py", line 219, in _start_build
  File "snapcraft\internal\remote_build\_worktree.py", line 325, in prepare_repository
  File "snapcraft\internal\remote_build\_worktree.py", line 244, in _process_part_sources
  File "snapcraft\internal\remote_build\_worktree.py", line 192, in _pull_source
  File "snapcraft\internal\sources\_local.py", line 41, in pull
  File "snapcraft\file_utils.py", line 216, in link_or_copy_tree
  File "ntpath.py", line 562, in relpath
ValueError: path is on mount 'C:', start on mount '\\\\wsl$\\Ubuntu'