Bump Linux leg timeout to 90min

[ViktorHofer]

To double check:

• The right tests are in and and the right validation has happened. Guidance: https://github.com/dotnet/core-eng/tree/master/Documentation/Validation

[ChadNedzlek]

This leg is weird, because it does two helix runs, sequentially, so it's doubly sensitive to any sort of helix issues.

[ChadNedzlek]

Which probably means the timeout should be a LOT higher.

[MattGal]

@ViktorHofer I'm also trying to investigate what's going on with the machines, but Azure has decided today's the day I learn about SSH keys and killed my ability to connect w/ passwords.