cerbero/bootstrap: Set and unset the build-tools env properly

When we cook the build-tools recipe, we temporarily override the
current environment with one derived from the build-tools
configuration, cook, and then re-set back to the old env.

We stopped doing this correctly after splitting out the 'fetch' step.
This caused a bootstrap failure because part of our build step was
using the wrong configuration.

cerbero/bootstrap/build_tools.py

@@ -72,6 +72,7 @@ class BuildTools (BootstrapperBase, Fetch):
             self.BUILD_TOOLS.append('glib-tools')
         self.BUILD_TOOLS += self.config.extra_build_tools
 
+    def _setup_env(self):
         # Use a common prefix for the build tools for all the configurations
         # so that it can be reused
         config = Config()
@@ -102,9 +103,12 @@ class BuildTools (BootstrapperBase, Fetch):
         self.recipes += self.PLAT_BUILD_TOOLS.get(self.config.platform, [])
 
     def start(self):
+        self._setup_env()
         oven = Oven(self.recipes, self.cookbook)
         oven.start_cooking()
         self.config.do_setup_env()
 
     def fetch_recipes(self):
+        self._setup_env()
         Fetch.fetch(self.cookbook, self.recipes, False, False, False, False)
+        self.config.do_setup_env()