GitHub actions: Do not unnecessarily install recommended packages

[tautschnig]

There is no need to install further packages beyond the ones explicitly
requested (or their dependencies) for our build steps. This fixes the
current build failure of Xen-build GitHub action, but also reduces the
time GitHub actions will take by a few seconds.

See https://github.com/diffblue/cbmc/pull/5677/checks?check_run_id=1583935711 for an example of the current build failures.

• 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 #5679 (9f94333) into develop (b1521c9) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop    #5679   +/-   ##
========================================
  Coverage    69.45%   69.45%           
========================================
  Files         1243     1243           
  Lines       100620   100620           
========================================
  Hits         69887    69887           
  Misses       30733    30733           

Flag	Coverage Δ
cproversmt2	43.13% <ø> (ø)
regression	66.34% <ø> (ø)
unit	32.28% <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 b1521c9...9f94333. Read the comment docs.