[FC] Fix context cancellation bug when reporting snapshot metadata (#11345)

The context is being canceled
[here](https://github.com/buildbuddy-io/buildbuddy/blob/master/enterprise/server/remote_execution/containers/firecracker/firecracker.go#L2501).
Defers run LIFO

There is a test for this bug (and an additional one) in this
[PR](https://github.com/buildbuddy-io/buildbuddy/pull/11302/changes#diff-095afcdc05618fde198c278a09b3c5f964cd16cd8eb0f6efbe9639f90b0697ff)

Author: maggie-lou

enterprise/server/remote_execution/containers/firecracker/firecracker.go

@@ -2400,6 +2400,9 @@ func (c *FirecrackerContainer) Exec(ctx context.Context, cmd *repb.Command, stdi
 	stage := "init"
 	result := &interfaces.CommandResult{ExitCode: commandutil.NoExitCode}
 	defer func() {
+		ctx, cancel = background.ExtendContextForFinalization(ctx, finalizationTimeout)
+		defer cancel()
+
 		// Attach VM metadata to the result
 		result.VMMetadata = c.getVMMetadata()
 		// Include whether the snapshot will be saved, so it can be displayed in the UI.