fixpath.sh changes broke _NT_SYMBOL_PATH in RunTests.gmk

A user reported that a crash on Windows is no longer including symbols in stack traces. Looking at the log, this message stands out:

[2021-01-12T18:27:58,764Z] _NT_SYMBOL_PATH=' Invalid switch - /cygdrive'

This indicates that the _NT_SYMBOL_PATH variable is not being setup correctly with the new fixpath.