Bump `nix` to 0.26.0

[elinorbgr]

Ah, looks like this bumps the MSRV?

[ids1024]

Hm, yeah, looks like it.

[ids1024]

Seems 1.56 is the oldest version that supports this, so might as well bump the MSRV to that. Wayland-rs is using 1.59, so I imagine there aren't any issues with this.

[codecov[bot]]

Codecov Report

Base: 89.52% // Head: 89.58% // Increases project coverage by +0.05% :tada:

Coverage data is based on head (82ea314) compared to base (df59c4a). Patch has no changes to coverable lines.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #122 +/- ## ========================================== + Coverage 89.52% 89.58% +0.05% ========================================== Files 17 17 Lines 1805 1805 ========================================== + Hits 1616 1617 +1 + Misses 189 188 -1 ``` | Flag | Coverage Δ | | |---|---|---| | macos-latest | `89.04% <ø> (-0.07%)` | :arrow_down: | | ubuntu-latest | `88.03% <ø> (ø)` | | 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. | [Impacted Files](https://codecov.io/gh/Smithay/calloop/pull/122?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Smithay) | Coverage Δ | | |---|---|---| | [src/sys/mod.rs](https://codecov.io/gh/Smithay/calloop/pull/122?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Smithay#diff-c3JjL3N5cy9tb2QucnM=) | `93.27% <0.00%> (+0.84%)` | :arrow_up: | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Smithay). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Smithay)

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