Fix a bug from #770 (need separate apt-get installs to access llvm-config) (#797)

Use a separate `apt-get install` for installing `FileCheck`, as the
detection needs `llvm-config`, which needs `llvm` to be installed first.

This fixes a bug from #770.

Author: kkysen

scripts/provision_deb.sh

@@ -47,18 +47,20 @@ packages=(
     zlib1g-dev
 )
 
+apt-get install -qq "${packages[@]}"
+
+# needs `llvm-config` from `llvm` package,
+# so the main packages need to be installed first
 if ! [[ -x "$(llvm-config --bindir)/FileCheck" ]]; then
 	IFS="." read -r major minor patch <<< "$(llvm-config --version)"
     if [[ ${major} -gt 6 ]]; then
         tools="llvm-${major}-tools"
     else
         tools="llvm-${major}.${minor}-tools"
     fi
-    packages+=("${tools}")
+    apt-get install -qq "${tools}"
 fi
 
-apt-get install -qq "${packages[@]}"
-
 apt-get clean # clear apt-caches to reduce image size
 
 python3 -m pip install --upgrade pip