CI fails with "failed to download"

[cmichi]

As much as I like our CI, I recently often have problems with jobs failing with this reason:

    Updating crates.io index
error: failed to download `libc v0.2.80`
Caused by:
  unable to get packages from source
Caused by:
  failed to parse manifest at `/ci-cache/ink/cargo/cmichi-add-transferred-balance-example/build-wasm/registry/src/github.com-1ecc6299db9ec823/libc-0.2.80/Cargo.toml`
Caused by:
  no targets specified in the manifest

It's mostly two crates for which it fails: libc v0.2.80 or paste v1.0.2.

Here are two jobs which failed recently:
https://gitlab.parity.io/parity/ink/-/jobs/701265
https://gitlab.parity.io/parity/ink/-/jobs/701277

Re-running them sometimes helps, but most often not. I just re-run from time to time and at one point it works.

Currently I have problems getting them to run for #555. I re-run them from time to time since yesterday, but they don't run through.

I talked to @TriplEight and he suspects a race-condition somewhere during the execution of pre_cache.sh.

[TriplEight]

It is the bug in cargo, CARGO_HOME more specifilally.

[TriplEight]

I'll try another approach to mitigate it, vendoring.

[Robbepop]

CARGO_HOME caching has just been removed. Hopefully fixing the bug. If not, then please re-open. Closed.