sources: generalize cache generation

Introduce a `setup` step in the workflow that is responsible of
generating the cache folder. This is then used in each download method.

sources/org.osbuild.curl

@@ -166,10 +166,8 @@ def download(items, cache):
 class CurlSource(sources.SourceService):
 
     def download(self, items, cache, _options):
-        cache = os.path.join(cache, "org.osbuild.files")
-        os.makedirs(cache, exist_ok=True)
-
-        download(items, cache)
+        self.setup(cache, "org.osbuild.files")
+        download(items, self.cache)
 
 
 def main():

sources/org.osbuild.inline

@@ -82,11 +82,9 @@ def process(items: Dict, cache: str, tmpdir):
 class InlineSource(sources.SourceService):
 
     def download(self, items, cache, _options):
-        cache = os.path.join(cache, "org.osbuild.files")
-        os.makedirs(cache, exist_ok=True)
-
+        self.setup(cache, "org.osbuild.files")
         with tempfile.TemporaryDirectory(prefix=".unverified-", dir=cache) as tmpdir:
-            process(items, cache, tmpdir)
+            process(items, self.cache, tmpdir)
 
 
 def main():

sources/org.osbuild.ostree

@@ -115,10 +115,9 @@ def download(items, cache):
 class OSTreeSource(sources.SourceService):
 
     def download(self, items, cache, _options):
-        cache = os.path.join(cache, "org.osbuild.ostree")
-        os.makedirs(cache, exist_ok=True)
+        self.setup(cache, "org.osbuild.ostree")
 
-        download(items, cache)
+        download(items, self.cache)
 
 
 def main():

sources/org.osbuild.skopeo

@@ -119,10 +119,9 @@ def download(items, cache):
 class SkopeoSource(sources.SourceService):
 
     def download(self, items, cache, _options):
-        cache = os.path.join(cache, "org.osbuild.containers")
-        os.makedirs(cache, exist_ok=True)
+        self.setup(cache, "org.osbuild.containers")
 
-        download(items, cache)
+        download(items, self.cache)
 
 
 def main():