Fix x64 perf run break

Fix x64 perf run break

After the checkin to add legacy JIT testing support for x86 we broke x64
runs.  THe issue is that we do not pass anything in to the TEST_ENV
variable on x64 and when a variable is empty and you attempt to use the
if exists syntax you fail.  I have added a nonsense file name that we
set this to before we go through the args loop. I was not able to find a
better solution and want to get the lab unblocked, but any batch masters
feel free to solve this the right way.

Commit migrated from https://github.com/dotnet/coreclr/commit/3ea4f8d694791b55b8ad088e93376b47d37e2410