workflows: Bump timeout of master GKE workflow
workflows: Bump timeout of master GKE workflow

The timeout was bumped to 75min already for the v1.12 version of the GKE
workflow, but not for the master version (cf. b6a5b5b26
("conformance-gke-v1.12: Miscellaneous fixes")).

The workflow is currently often failing with a timeout on the last of
the four connectivity test runs. Each connectivity test takes between 13
and 14min on GKE. Adding 15min to the workflow should therefore be
enough to finish.

Signed-off-by: Paul Chaignon <a027184a55211cd23e3f3094f1fdc728df5e0500@cilium.io>

cilium/cilium,cilium/cilium,cilium/cilium,cilium/cilium,cilium/cilium