ci: fix clang-13 installation

For some reason Ubuntu Focal repositories now have `llvm-13` virtual
package which can't be installed, but successfully fools our check,
resulting in no clang/llvm being installed...

```
$ apt show llvm-13
Package: llvm-13
State: not a real package (virtual)
N: Can't select candidate version from package llvm-13 as it has no candidate
N: Can't select versions from package 'llvm-13' as it is purely virtual
N: No packages found

$ apt install --dry-run llvm-13
Reading package lists... Done
Building dependency tree
Reading state information... Done
Package llvm-13 is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source

E: Package 'llvm-13' has no installation candidate
```

(cherry picked from commit b491d74064f9d5e17a71b38b014434237169a077)

diff --git a/.github/workflows/build_test.sh b/.github/workflows/build_test.sh
index 549e59b2c927fdd2806b8262fa102d68d045ed17..5a173a18d50ab948a718614b84aa25b2fec67285 100755 (executable)
--- a/.github/workflows/build_test.sh
+++ b/.github/workflows/build_test.sh
@@ -78,12 +78,12 @@ if [[ "$COMPILER" == clang ]]; then
     # ATTOW llvm-11 got into focal-updates, which conflicts with llvm-11
     # provided by the apt.llvm.org repositories. Let's use the system
     # llvm package if available in such cases to avoid that.
-    if ! apt show --quiet "llvm-$COMPILER_VERSION" &>/dev/null; then
+    if ! apt install --dry-run "llvm-$COMPILER_VERSION" >/dev/null; then
         # Latest LLVM stack deb packages provided by https://apt.llvm.org/
         # Following snippet was partly borrowed from https://apt.llvm.org/llvm.sh
         wget -O - https://apt.llvm.org/llvm-snapshot.gpg.key | gpg --yes --dearmor --output /usr/share/keyrings/apt-llvm-org.gpg
         printf "deb [signed-by=/usr/share/keyrings/apt-llvm-org.gpg] http://apt.llvm.org/%s/   llvm-toolchain-%s-%s  main\n" \
-               "$RELEASE" "$RELEASE" "$COMPILER_VERSION" >/etc/apt/sources.list.d/llvm-toolchain.list
+               "$RELEASE" "$RELEASE" "$COMPILER_VERSION" >/etc/apt/sources.list.d/llvm-toolchain.list
         PACKAGES+=("clang-$COMPILER_VERSION" "lldb-$COMPILER_VERSION" "lld-$COMPILER_VERSION" "clangd-$COMPILER_VERSION")
     fi
 elif [[ "$COMPILER" == gcc ]]; then