Added needed flag to pacman invocation to only install packages that …
diff --git a/bootstrap/archlinux.sh b/bootstrap/archlinux.sh
index 6de7c23d4ec..884262f0b7f 100755
--- a/bootstrap/archlinux.sh
+++ b/bootstrap/archlinux.sh
@@ -3,7 +3,7 @@
 # "python-virtualenv" is Python3, but "python2-virtualenv" provides
 # only "virtualenv2" binary, not "virtualenv" necessary in
 # ./bootstrap/dev/_common_venv.sh
-pacman -S \
+pacman -S --needed \
   git \
   python2 \
   python-virtualenv \

…are not already installed.

This makes for a better user experience on Arch.
