fix borrow error when scheduling future inside an executor's callback

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (c6f6b88) 86.30% compared to head (ba35762) 86.40%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #165 +/- ## ========================================== + Coverage 86.30% 86.40% +0.09% ========================================== Files 16 16 Lines 2008 2008 ========================================== + Hits 1733 1735 +2 + Misses 275 273 -2 ``` | [Flag](https://app.codecov.io/gh/Smithay/calloop/pull/165/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Smithay) | Coverage Δ | | |---|---|---| | [macos-latest](https://app.codecov.io/gh/Smithay/calloop/pull/165/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Smithay) | `85.51% <100.00%> (+0.10%)` | :arrow_up: | | [ubuntu-latest](https://app.codecov.io/gh/Smithay/calloop/pull/165/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Smithay) | `85.98% <100.00%> (+0.10%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Smithay#carryforward-flags-in-the-pull-request-comment) to find out more.

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[elinorbgr]

Thanks!