Fetch jobs resume despite "terminate" option on CTRL-C

Summary

When first attempting to terminate buildstream mid fetch, buildstream appears to continue/restart the fetch operation.

Steps to reproduce

• cd into doc/examples/flatpak-autotools
• Ensure your ~/.cache/buildstream/artifacts is cleared
• Run bst build hello.bst
• Wait for fetch:base/sdk.bst to start
• Press CTRL-C followed by t to terminate
  • Observe as the STATUS messages begin to start coming in again
• Repeating the previous step will terminate the session as expected.

What is the current bug behavior?

BuildStream fails to terminate during a fetch job on the first attempt.

What is the expected correct behavior?

BuildStream should terminate any ongoing jobs and exit as specified.

Relevant logs and/or screenshots

Possible fixes

Other relevant information

• BuildStream version affected: master

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