test-rebase-interruptions: stabilize for Windows External hooks end up launching cmd.exe, which knows nothing about $VAR syntax. For some reason, I thought that Mercurial would substitute in the value, in order to paper over the platform difference. But I can't find that in the documentation, and there's at least one other use of this pattern [1]. [1] https://www.mercurial-scm.org/repo/hg/file/tip/tests/test-histedit-fold.t#l477