l3build: Incorrect dir separator

The idea of a system-dependent one got dropped.


git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@5142 de43f980-851b-0410-b2f7-c40aca1f87e0
l3build: Temporarily use "regression-test.tex" from support

Something is up with the installation of deps: not sure what at present, and
have other things to check in!


git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@5143 de43f980-851b-0410-b2f7-c40aca1f87e0