build(deps): update nix requirement from ^0.20 to ^0.21

[dependabot-preview]

Updates the requirements on nix to permit the latest version.

Changelog

Sourced from nix's changelog.

[0.21.0] - 31 May 2021

Added

• Added getresuid and getresgid (#1430)
• Added TIMESTAMPNS support for linux (#1402)
• Added sendfile64 (#1439)
• Added MS_LAZYTIME to MsFlags (#1437)

Changed

• Made forkpty unsafe, like fork (#1390)
• Made Uid, Gid and Pid methods from_raw and as_raw a const fn (#1429)
• Made Uid::is_root a const fn (#1429)
• AioCb is now always pinned. Once a libc::aiocb gets sent to the kernel, its address in memory must not change. Nix now enforces that by using std::pin. Most users won't need to change anything, except when using aio_suspend. See that method's documentation for the new usage. (#1440)
• LioCb is now constructed using a distinct LioCbBuilder struct. This avoids a soundness issue with the old LioCb. Usage is similar but construction now uses the builder pattern. See the documentation for details. (#1440)
• Minimum supported Rust version is now 1.41.0. (#1440)

Fixed

• Allow sockaddr_ll size, as reported by the Linux kernel, to be smaller then it's definition (#1395)
• Fix spurious errors using sendmmsg with multiple cmsgs (#1414)

Removed

• Removed sys::socket::accept4 from Android arm because libc removed it in version 0.2.87. (#1399)
• AioCb::from_boxed_slice and AioCb::from_boxed_mut_slice have been removed. They were useful with earlier versions of Rust, but should no longer be needed now that async/await are available. AioCbs now work exclusively with borrowed buffers, not owned ones. (#1440)

[0.20.0] - 20 February 2021

Added

... (truncated)

Commits

• db2af19 Merge pull request #1444 from asomers/r0.21.0
• ec1a124 Release v0.21.0
• 478db7d Merge #1430
• b481a0e Add getresuid() and getresgid() to unistd
• 77e4402 Merge #1443
• 7b7d954 misc Clippy cleanup
• a55dd45 Merge #1437
• 50e6b22 Add MS_LAZYTIME flag
• 892daf1 Merge #1440
• 5ac876e Adapt aio to the world of async/await, and fix some potential unsoundness.
• Additional commits viewable in compare view

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[dependabot-preview]

Superseded by #339.