mkosi: Stop passing package environment variables to tools image

The tools image is not guaranteed to be the same distribution as the
target distribution and so might have different package environment
variables than the main image yet we currently unconditionally use the
same package environment variables for both of them.

Let's fix this by not passing the package environment variables to the
tools image and subimages anymore, and instead having the main, tools and
build images separately include a config file with the required environment
variables.

tools/fetch-distro.py

@@ -127,7 +127,7 @@ def update_distro(args, distro: str, config: dict):
     changes = subprocess.check_output(cmd, text=True).strip()
 
     conf_dir = Path('mkosi.conf.d')
-    files = conf_dir.glob('*/*.conf')
+    files = conf_dir.glob('**/pkgenv.conf')
     for file in files:
         s = file.read_text()
         if old_commit in s: