nss: set C++ compiler (#29195)

var/spack/repos/builtin/packages/nss/package.py

@@ -38,6 +38,7 @@ def build_targets(self):
         # We cannot use nss_build_all because this will try to build nspr.
         targets = ['all', 'latest']
 
+        targets.append('CCC={}'.format(spack_cxx))
         targets.append('USE_64=1')
         targets.append('BUILD_OPT=1')