theia: clean up test/dummy/software.cfg

In addition to update version of slapos.recipe.build, this fixes the
following misuse:
- init phase must only initialize the section, which implies no change
  on the file system
- init/install/update's slapos.recipe.build should not contain buildout
  substitutions (theoritically, there could be wrong escaping,
  e.g. ${buildout:directory} containing quotes)
These 2 points didn't not matter here but it gave a bad example
for anyone learning buildout.

software/theia/test/dummy/software.cfg

@@ -9,11 +9,12 @@ parts =
 
 [log-env]
 recipe = slapos.recipe.build
-init =
+path = ${buildout:directory}/../env.json
+update =
   import json
   import os
-  with open("${buildout:directory}/../env.json", 'w') as f:
+  with open(options['path'], 'w') as f:
     json.dump(dict(os.environ), f)
 
 [versions]
-slapos.recipe.build = 0.47
+slapos.recipe.build = 0.48