CI: Upgrade coverage action

[mkroening]

Closes #206.

[codecov]

Codecov Report

Merging #208 (cc2578c) into master (71ac00c) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #208   +/-   ##
=======================================
  Coverage   20.49%   20.49%           
=======================================
  Files          17       17           
  Lines        2654     2654           
=======================================
  Hits          544      544           
  Misses       2110     2110           

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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