Xen-build GitHub action: use ccache and Ubuntu 20.04

[tautschnig]

The CBMC build in this action takes more than 10 minutes (building just
some selected directories), when the full build using ccache takes less
than 1 minute. Use ccache to achieve the same speed-up. As we only have
a make-build cache for 20.04, upgrade the environment to this Ubuntu
version.

• 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).
• 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.

[codecov]

Codecov Report

Merging #5699 (a16b8e7) into develop (8ea67e7) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop    #5699   +/-   ##
========================================
  Coverage    69.50%   69.50%           
========================================
  Files         1243     1243           
  Lines       100674   100674           
========================================
  Hits         69977    69977           
  Misses       30697    30697           

Flag	Coverage Δ
cproversmt2	43.14% <ø> (ø)
regression	66.40% <ø> (ø)
unit	32.25% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 8ea67e7...a16b8e7. Read the comment docs.

[NlightNFotis]

LGTM