Upgrade nix to release

[mkroening]

bors r+

[codecov]

Codecov Report

Merging #218 (ab41444) into master (b13a88e) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #218   +/-   ##
=======================================
  Coverage   20.45%   20.45%           
=======================================
  Files          17       17           
  Lines        2659     2659           
=======================================
  Hits          544      544           
  Misses       2115     2115           

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[bors]

Build succeeded:

• Build (macos-latest)
• Build (ubuntu-latest)
• Cargo tests (macos-latest)
• Cargo tests (ubuntu-latest)
• Clippy
• Format
• KVM Tests
• Rusty-Hermit Kernel based integration tests