Allow skipping apt update

- environment variable TRAVIS_SKIP_APT_UPDATE can be set to non-empty to
  skip apt update when running travis2bash.sh

travis2bash.sh

@@ -65,8 +65,10 @@ function aptinstall() {
     # apt install
     # shellcheck disable=SC2154
     if [[ "$(declare -p travis_addons_apt_packages)" =~ "declare -a" ]]; then
-        echo -e "\nsudo apt -y update\n"
-        sudo apt -y update
+        if [ -z "$TRAVIS_SKIP_APT_UPDATE" ]; then
+            echo -e "\nsudo apt -y update\n"
+            sudo apt -y update
+        fi
         for ((i = 0; i < ${#travis_addons_apt_packages[@]}; i++))
         do
             echo -e "\napt-get -yq --no-install-suggests --no-install-recommends install ${travis_addons_apt_packages[$i]}\n"