import --force is broken on Windows

[rotu]

vcs import --force fails with a PermissionError on Windows.
This is because git creates its index files with the readonly flag, which must be cleared before the file can be deleted.

Invocation of command 'import' on client 'git' failed: PermissionError: [WinError 5] Access is denied: 'src/eProsima/Fast-CDR' (c:\python37\lib\site-packages\vcstool\clients\git.py:242)

https://github.com/ros2/rmw_cyclonedds/pull/168/checks?check_run_id=627448665#step:4:430

[dirk-thomas]

Thanks for the report. The problem has been addressed in #134.