[libcxx] [test] Remove --env PATH from the static clangcl config

The PATH is set in order to be able to find the tested DLL at runtime.
When linking statically, it's not necessary to set the PATH.

Setting PATH in the executor has the downside that it clears the
existing path (it's not prepended/appended to it), which means
that the executed tools can't find other tools - which sets the
executor-has-no-bash flag.

By removing the unnecessary setting of PATH, we have a properly
working bash even when wrapped by the executor, which gets rid
of the executor-has-no-bash flag in this test configuration, which
makes 9 more testcases be executed.

Differential Revision: https://reviews.llvm.org/D131481