Fix mill for Windows

[ArmborstL]

Checklist:

• No wiki changes required

PR description

A recent commit by @superaxander added quotes to file paths to allow spaces in paths. However, on my Windows machine, the quotes in the .classpath file resulted in Error: Could not find or load main class vct.main.Main, so we removed those quotes again.
The CI did not catch this, because https://github.com/utwente-fmt/vercors/blob/dev/.github/workflows/scalatest.yml uses a release jar, bypassing the classpath file.

ToDo:

• figure out how to integrate paths with spaces into the classpath
  • wrapping each path in the classpath file individually in quotes also does not help
• add a test to the CI? (or are we fine with dev having problems as long as the release works?)

[bobismijnnaam]

Maybe one more version of quoting that we can try: https://www.digizol.com/2006/12/setting-java-classpath-option-with.html . So quoting individual elements of the classpath.

java -classpath "C:/Documents and Settings/user/project/lib/axis.jar";"C:/Documents and Settings/user/project/lib/axis-ant.jar" TestClient

Haven't tested it myself...

[ArmborstL]

As @superaxander figured out: without quotes, the Windows paths in the classpath file are used unchanged, i.e. the \ path separator remains unchanged. The moment you quote it, the parser treats \ as an escape character, mangling up the Windows paths. So the solution is to replace all \ in all the paths in classpath with \\.