Replace actions-rs/toolchain by direct command

[tautschnig]

actions-rs/toolchain uses deprecated GitHub commands and a deprecated node version. There really is no need for such a dependency, a simple direct command does the trick.

• Each commit message has a non-empty body, explaining why the change was made.
• n/a Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• n/a The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• n/a My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[NlightNFotis]

A number of CI checks failing because of networking/package manager issues unrelated to this PR and its changes.

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: -0.01 ⚠️

Comparison is base (8c152bc) 78.53% compared to head (772a1d5) 78.53%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #7713      +/-   ##
===========================================
- Coverage    78.53%   78.53%   -0.01%     
===========================================
  Files         1686     1686              
  Lines       192930   192930              
===========================================
- Hits        151520   151519       -1     
- Misses       41410    41411       +1     

see 1 file with indirect coverage changes

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