Increase bors timeout from 2h to 3h

It seems hydra is often slow to schedule/run the jobs. (≈18% of bors r+ it seems)

It should be better to increase the timeout to 3h, than to have it fail.

This doesn't affect the timeout of buildkite or hydra themselves.

bors.toml

@@ -2,6 +2,6 @@ status = [
   "buildkite/cardano-wallet",
   "ci/hydra-build:required",
 ]
-timeout_sec = 7200
+timeout_sec = 10800
 required_approvals = 1
 delete_merged_branches = false