guix: warn SOURCE_DATE_EPOCH set in guix-codesign

Currently there is a warning for this in guix-build, but we also need
one in guix-codesign, otherwise the codesigned hashes are not
reproducible.

Move common functionality into prelude and call the function in both
guix actions.

contrib/guix/guix-codesign

@@ -67,6 +67,12 @@ EOF
 exit 1
 fi
 
+################
+# SOURCE_DATE_EPOCH should not unintentionally be set
+################
+
+check_source_date_epoch
+
 ################
 # The codesignature git worktree should not be dirty
 ################