Add fontconfig to debian apt deps, make fc-cache non-fatal

Discovered on crouton bootstrap: a fresh debian install may not
have fontconfig installed, so fc-cache is missing. The script's
font install step crashed at fc-cache, aborting the rest of the
bootstrap (cargo install bat never ran).

Fixes:
1. Add fontconfig to APT_PKGS so fc-cache and fc-list are present
   on debian from the start (matches what's typically pre-installed
   on most desktop distros but not on minimal server installs)
2. Wrap fc-cache in 'command -v' guard so if the package somehow
   isn't installed, the script logs a warning and continues instead
   of aborting the whole chain

run_once_20-install-user-packages.sh.tmpl

@@ -49,6 +49,7 @@ APT_PKGS=(
     unzip p7zip
     openssh-client
     ca-certificates curl wget
+    fontconfig
 )
 
 log "installing apt packages"
@@ -177,8 +178,12 @@ else
         log "extracting font files"
         unzip -q -o "$TMP_ZIP" -d "$FONT_DIR"
         rm -f "$TMP_ZIP"
-        log "running fc-cache"
-        fc-cache -fv >/dev/null
+        log "refreshing font cache (fc-cache)"
+        if command -v fc-cache >/dev/null 2>&1; then
+            fc-cache -fv >/dev/null
+        else
+            log "WARNING: fc-cache not found — install fontconfig package"
+        fi
     else
         log "WARNING: failed to download Maple Mono NF — nvim will warn about missing font"
         log "manual install: https://github.com/subframe7536/Maple-font"