[clang] [test] Skip a test that sets PATH on Windows (#95096)

The same has been done in a couple other existing tests, that also are
skipped on Windows (e.g. ld-path.c). Some tests that really do want to
test setting the path on Windows does it differently, see e.g.
ps4-ps5-linker-win.c.

Since a65771fc, the spirv-toolchain.cl
test does one test where PATH is set. Setting PATH does work in some
build configurations - however, if built with e.g. llvm-mingw, the built
Clang executable depends on libc++.dll (and libunwind.dll) which are
found in PATH. If the PATH is overridden, the newly built Clang
executable no longer can run.