SERVER-5702 Fix Build Break

SCons has decidedly mixed feelings about recursively substituting expansion
variables in env.Alias() statements.  Replace "$UNITTEST_ALIAS" with "unittests"
in SConstruct.

Author: Andy Schwerin

SConstruct

@@ -1087,4 +1087,4 @@ def clean_old_dist_builds(env, target, source):
 env.Alias("dist_clean", [], [clean_old_dist_builds])
 env.AlwaysBuild("dist_clean")
 
-env.Alias('all', ['core', 'tools', 'clientTests', 'test', '$UNITTEST_ALIAS'])
+env.Alias('all', ['core', 'tools', 'clientTests', 'test', 'unittests'])