use 'sed' instead of 'rm;git-checkout' to fix CRLF line endings in Cygwin CI

use 'sed' instead of 'rm;git-checkout' to fix CRLF line endings in Cygwin CI

Only selective artifacts in the repository are sensitive to line endings. E.g.
C/C++ and Python code is happily parsed by the compiler/interpreter regardless
of which line ending style it uses.

This should hopefully accelerate the Cygwin CI tasks slightly. The assumption is
that running `sed` on a few files should be faster than deleting all files and
recreating them.