cerbero: Fix wget errors while running fetch-bootstrap

wget: [...]/dist/linux_x86_64/lib/libgnutls.so.30: version `GNUTLS_3_6_3' not found (required by wget)

We need config._pre_environ to be the env that was set when Cerbero
was called. But while bootstrapping build tools, we instantiate that
again without resetting the env to that value, so it picks up the
current Cerbero env and sets it as the _pre_environ.

Clear the env and set it to the original env before instantiating
Config()

cerbero/bootstrap/build_tools.py

@@ -70,11 +70,12 @@ class BuildTools (BootstrapperBase, Fetch):
         self.BUILD_TOOLS += self.config.extra_build_tools
 
     def _setup_env(self):
+        # Start with the original env that cerbero was called with
+        os.environ.clear()
+        os.environ.update(self.config._pre_environ)
         # Use a common prefix for the build tools for all the configurations
         # so that it can be reused
         config = Config()
-        os.environ.clear()
-        os.environ.update(self.config._pre_environ)
         config.prefix = self.config.build_tools_prefix
         config.home_dir = self.config.home_dir
         config.local_sources = self.config.local_sources