rust: fix file folding map The file folding map, frequently used on macOS, had two issues: * the means for converting it to Python didn't work * a minor typo when copying the python code, where `!=` became `==` With this, the rust code passes all tests on macOS. Test Plan: I'm currently doing a full test run on a case-insensitive file system. If it passes, perhaps we can change the platform check from an error to a warning? Differential Revision: https://phab.mercurial-scm.org/D9671

test: replace a many occurence of `python` with `$PYTHON` Otherwise this can use the wrong python version, or worse, not find any python at all. Differential Revision: https://phab.mercurial-scm.org/D9730