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
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