perf: ensure HGRCPATH is exported on Windows
After dropping the garbage collector hack, `hg perfstartup` started yelling
about not being able to import the evolve extension, which I have in my user
config. Launching `env` shows that an empty HGRCPATH isn't exported to the
environment. Since `env` doesn't quote, I have no idea if the variable is
trimmed, but Mercurial doesn't complain when processing it.
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