feat: Kill spot instance requests

[spalladino]

Cancels spot instance requests at the end of spot_run_script to prevent them from counting against quota. We have been getting the following error in our CI:

An error occurred (MaxSpotInstanceCountExceeded) when calling the RequestSpotInstances operation: Max spot instance count exceeded

And this answer hints at spot requests not closed. So let's see if this fixes it.

[ludamad]

Not sure if it applies but a trap is something to consider, it's similar to a BASH finally block (normal execution or a sudden error will both run it)

#!/bin/bash
function on_exit {
    if [ -f "sir-$COMMIT_HASH:$JOB_NAME.txt" ]; then
        SIR=$(cat sir-$COMMIT_HASH:$JOB_NAME.txt)
        echo "Cancelling spot instance request $SIR"
        aws ec2 cancel-spot-instance-requests --spot-instance-request-ids $SIR > /dev/null
    fi
}
trap on_exit EXIT

[spalladino]

I like the idea of using a trap, you should've just pushed it on top of this!