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.
$ hg init
$ echo This is file a1 > a
$ hg add a
$ hg commit -m "commit #0"
$ ls
a
$ echo This is file b1 > b
$ hg add b
$ hg commit -m "commit #1"
$ hg co 0
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
B should disappear
$ ls
a