Changed rusty-demo workflow as it interferences with workspaces

[jounathaen]

Separated this from #452.

See the discussion here: #452 (comment)

[codecov]

Codecov Report

Merging #457 (ef13e54) into master (7455ee8) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #457   +/-   ##
=======================================
  Coverage   63.46%   63.46%           
=======================================
  Files          16       16           
  Lines        2206     2206           
=======================================
  Hits         1400     1400           
  Misses        806      806           

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[mkroening]

Untouched run_images job

• Runs pre-build images of Rust and C apps.
• 👍 Fast (no image building necessary)
• 👍 Easy to modify (just replace the images)
• 👍 Stable (images don't change implicitly)
• (👎) Little test (Currently runs only very simple hello world apps)
  • Easy to change!

Old rusty_demo job

• Runs freshly built images of RustyHermit's repositories rusty_demo example.
• 👎 Slow (image building necessary)
• 👎 Hard to modify (sources are in external repo)
• 🤷 Unstable (tracks latest hermit-sys git)
  • 👎 images change without changing the Uhyve repo
  • 👍 Detects failure before Uhyve and hermit-sys releases
    • Might not be Uhyve's task
• 👍 Big test (Runs most complete demo test suite)

Proposed rusty_demo job

• 👎 Slow (image building necessary)
• 👎 Hard to modify (sources are in external repo)
• 👎 Unstable (tracks latest rusty-demo git)
  • Runs already released hermit-sys
• 👎 Small test (rusty-demo is only a most minimal example)

I propose that instead of changing the rusty_demo job in this way, we either leave it as is or remove it completely and instead use a more sophisticated image in the run_images job, depending on whether we want to test against hermit-sys git with Uhyve git.