[tex] Skip Arch install if already present

recipes/tex

@@ -12,6 +12,9 @@ case "$_PLATFORM" in
         _brew_cask mactex
         ;;
     arch)
+        # Install texlive if it isn't already available (e.g. a local or
+        # portable installation).
+        if ! command -v tlmgr >/dev/null; then
             _recipe _arch
             _pacman texlive-bibtexextra
             _pacman texlive-core
@@ -29,6 +32,9 @@ case "$_PLATFORM" in
             __tex_init=""
             __tex_info="tllocalmgr info --localsearch"
             __tex_install="yes | tllocalmgr install"
+        else
+            echo "[tex] Texlive appears to already be installed, skipping."
+        fi
         ;;
     debian)
         _recipe _apt