Disable nightly builds on the forks

[IAmSurajBobade]

Disable nightly builds on the forks, consuming avoidable resources.

Note: This issue is not to related to fixing current nighly checks.

Possible solution:
We can disable the nightly runs to only run it on primary repository using following condition

jobs:
  vulncheck:
    if: github.event_name != 'schedule' || github.repository == 'modelcontextprotocol/go-sdk'
    # ... other fields

  conformance:
    if: github.event_name != 'schedule' || github.repository == 'modelcontextprotocol/go-sdk'
    # ... other fields