Store the default Spack environment in the `build_system` instance

[jjotero]

With fixtures, one can create a Spack installer test-wrapper that may be invoked multiple times to install different specs. Hence, the parent test that uses the fixture should have a way to retrieve the default value of the Spack environment used.

class MyTest(rfm.RunOnlyRegressionTest):
    ...
    zlib_install = fixture(SpackInstallWrapper, variables={'specs:'['zlib']})

    @run_before('run'):
    def retrieve_spack_env(self):
        env = self.zlib_install.build_system.environment