bots: Fix data type in tests-policy
bots: Fix data type in tests-policy

When opening the flake data fails, provide a byte array instead of a
string as fallback, so that the subsequent `.decode()` call doesn't fail
on an `AttributeError`.

Closes #1696
cockpit-project/cockpit,cockpit-project/cockpit,moraleslazaro/cockpit,moraleslazaro/cockpit,deryni/cockpit,martinpitt/cockpit,deryni/cockpit,moraleslazaro/cockpit,cockpituous/cockpit,cockpituous/cockpit,cockpit-project/cockpit,mvollmer/cockpit,martinpitt/cockpit,garrett/cockpit,moraleslazaro/cockpit,martinpitt/cockpit,cockpituous/cockpit,cockpituous/cockpit,deryni/cockpit,deryni/cockpit,moraleslazaro/cockpit,martinpitt/cockpit,cockpit-project/cockpit,martinpitt/cockpit,moraleslazaro/cockpit,garrett/cockpit,deryni/cockpit,cockpit-project/cockpit,garrett/cockpit,mvollmer/cockpit,moraleslazaro/cockpit,garrett/cockpit,deryni/cockpit,deryni/cockpit,mvollmer/cockpit,garrett/cockpit,cockpituous/cockpit,mvollmer/cockpit,cockpituous/cockpit,mvollmer/cockpit