rpmmd: pass in cache directory explicitly

rpmmd looked at the CACHE_DIRECTORY environment variable to set a path
for the dnf repository cache.  Aside from being a smelly thing to do
from a library, this breaks osbuild-pipeline and osbuild-dnf-json-tests,
which don't run as systemd services and thus don't have CACHE_DIRECTORY
set.

Explicitly pass the cache directory to rpmmd. Keep using a path based on
CACHE_DIRECTORY for osbuild-composer. Use the user's `.cache` directory
for osbuild-pipeline and a temporary directory for the tests.

cmd/osbuild-dnf-json-tests/main.go

@@ -57,7 +57,7 @@ func TestFetchChecksum(quiet bool, dir string) {
 	if !quiet {
 		log.Println("Running TestFetchChecksum on:", dir)
 	}
-	rpmMetadata := rpmmd.NewRPMMD()
+	rpmMetadata := rpmmd.NewRPMMD(path.Join(dir, "rpmmd"))
 	_, c, err := rpmMetadata.FetchMetadata([]rpmmd.RepoConfig{repoCfg}, "platform:f31")
 	if err != nil {
 		log.Panic("Failed to fetch checksum:", err)