software/slapos-sr-testing: fix python3 profile

It got broken in 3a1e5628 (software/slapos-sr-testing: use nxdtest, 2020-10-28)

software/slapos-sr-testing/software.cfg

@@ -169,7 +169,8 @@ setup = ${slapos.core-repository:location}
 
 [eggs]
 <= python-interpreter
-eggs +=
+eggs =
+  ${python-interpreter:eggs}
   ${lxml-python:egg}
   ${slapos.core-setup:egg}
   ${pillow-python:egg}