rust-toolchain.toml: Default to stable channel

[mkroening]

Closes #247.

[codecov]

Codecov Report

Merging #254 (7d514c8) into master (57bbc3c) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #254   +/-   ##
=======================================
  Coverage   62.58%   62.58%           
=======================================
  Files          16       16           
  Lines        2475     2475           
=======================================
  Hits         1549     1549           
  Misses        926      926           

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[mkroening]

bors r+

[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