Don't copy host cross variants to testcases

Checking for os == BuildOS isn't sufficient if we are also cross
compiling to other architectures for that OS, for example when
BuildOS is linux_musl and we are building for x86_64 and cross
compiling to arm64.  Also skip variants that are HostCross.

Bug: 236052820
Test: linux musl x86_64 + arm64 build
Change-Id: I58ce0d8cbfa7c3e209d22c03e49f6d094ff5e107
1 file changed