Use 'os.uname().machine' to get machine architecture instead of 'unam…

…e -i'

uname -i returns unknown.

Resolves: rhbz#2248930

pyanaconda/modules/payloads/payload/dnf/installation.py

@@ -360,7 +360,7 @@ def run(self):
 
         # Get substitutions for variables.
         # TODO: replace the interpolation with DNF once possible
-        basearch = util.execWithCapture("uname", ["-i"]).strip().replace("'", "")
+        basearch = os.uname().machine
         releasever = util.get_os_release_value("VERSION_ID", sysroot=self._sysroot) or ""
 
         # Import GPG keys to RPM database.