GitHub actions: ensure gdb remains installed on Ubuntu 18.04

[tautschnig]

Removing GCC 10 previously resulted in uninstalling gdb, which makes
memory-analyzer tests fail.

• 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.

[codecov]

Codecov Report

Merging #6061 (6036d6d) into develop (a22fb03) will increase coverage by 0.01%.
The diff coverage is 82.17%.

@@             Coverage Diff             @@
##           develop    #6061      +/-   ##
===========================================
+ Coverage    73.90%   73.92%   +0.01%     
===========================================
  Files         1444     1444              
  Lines       157448   157481      +33     
===========================================
+ Hits        116368   116415      +47     
+ Misses       41080    41066      -14     

Impacted Files	Coverage Δ
src/goto-instrument/code_contracts.h	94.11% <ø> (ø)
src/solvers/flattening/boolbv.cpp	80.00% <0.00%> (ø)
src/solvers/flattening/boolbv_case.cpp	0.00% <0.00%> (ø)
src/solvers/flattening/boolbv_cond.cpp	0.00% <0.00%> (ø)
src/solvers/prop/prop.cpp	42.85% <0.00%> (ø)
src/solvers/prop/prop.h	84.37% <ø> (ø)
src/solvers/sat/cnf.h	83.33% <ø> (ø)
src/util/irep.h	93.47% <ø> (ø)
src/util/std_expr.h	92.08% <ø> (ø)
src/ansi-c/c_typecheck_initializer.cpp	64.98% <44.44%> (ø)
... and 32 more

---

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[NlightNFotis]

LGTM