GitHub actions: ensure gdb remains installed on Ubuntu 18.04

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

Author: tautschnig

.github/workflows/pull-request-checks.yaml

@@ -461,7 +461,7 @@ jobs:
           # remove libgcc-s1, which isn't normally available in Ubuntu 18.04
           target=$(dpkg-query -W --showformat='${Version}\n' gcc-8-base | head -n 1)
           # libgcc1 uses an epoch, thus the extra 1:
-          sudo apt-get install -y --allow-downgrades --reinstall gcc g++ libgcc-s1- libstdc++6=$target liblsan0=$target libtsan0=$target libcc1-0=$target libgcc1=1:$target
+          sudo apt-get install -y --allow-downgrades --reinstall gcc g++ libgcc-s1- libstdc++6=$target liblsan0=$target libtsan0=$target libcc1-0=$target libgcc1=1:$target gdb=8.1.1-0ubuntu1
       - name: Confirm z3 solver is available and log the version installed
         run: z3 --version
       - name: Prepare ccache

.github/workflows/release-packages.yaml

@@ -76,7 +76,7 @@ jobs:
           # remove libgcc-s1, which isn't normally available in Ubuntu 18.04
           target=$(dpkg-query -W --showformat='${Version}\n' gcc-8-base | head -n 1)
           # libgcc1 uses an epoch, thus the extra 1:
-          sudo apt-get install -y --allow-downgrades --reinstall gcc g++ libgcc-s1- libstdc++6=$target liblsan0=$target libtsan0=$target libcc1-0=$target libgcc1=1:$target
+          sudo apt-get install -y --allow-downgrades --reinstall gcc g++ libgcc-s1- libstdc++6=$target liblsan0=$target libtsan0=$target libcc1-0=$target libgcc1=1:$target gdb=8.1.1-0ubuntu1
       - name: Confirm z3 solver is available and log the version installed
         run: z3 --version
       - name: Prepare ccache