ci: Cache downloaded nw.js tarball

This saves time and bandwidth.

Closes jwilkes/purr-data#668

.gitlab-ci.yml

 # This file creates automated tests on GitLab CI for Purr Data.
 # See https://docs.gitlab.com/ee/ci/yaml/README.html for details of the format.
 
+cache:
+  key: global-cache
+
 # These are script snippets used in the jobs defined below.
+.fetch_nwjs: &fetch_nwjs
+  - nwjs_filename=$(l2ork_addons/nwjs_version_for_platform.sh)
+  - if [ -e /cache/$nwjs_filename ]; then cp /cache/$nwjs_filename ./l2ork_addons ; fi
+
 .debian_image_preparation: &debian_image_preparation
   - echo "" | sudo -S apt-get -qq purge -y pd-l2ork > /dev/null || echo "couldn't purge..."
   - echo "" | sudo -S DEBIAN_FRONTEND=noninteractive apt-get -qq -y update > /dev/null
@@ -12,6 +19,7 @@
   - cd l2ork_addons
   - export V=0
   - ./tar_em_up.sh $BUILD_TYPE_FLAG
+  - cp nwjs-* /cache/
   - cd ..
   - echo -e "section_end:`date +%s`:build_all\r\e[0K"
 
@@ -46,6 +54,7 @@ debian_stretch_x86_64_deb:
      BUILD_TYPE_FLAG: -B
    before_script:
      - *debian_image_preparation
+     - *fetch_nwjs
    script:
      - *build_all
      - *test_linux
@@ -64,6 +73,7 @@ ubuntu_16.04_x86_64_deb:
      BUILD_TYPE_FLAG: -B
    before_script:
      - *debian_image_preparation
+     - *fetch_nwjs
    script:
      - *build_all
      - *test_linux
@@ -80,6 +90,8 @@ osx_10.11_x86_64_dmg:
      - dmg
    variables:
      BUILD_TYPE_FLAG: -X
+   before_script:
+     - *fetch_nwjs
    script:
      - echo `pwd`
      - *build_all
@@ -97,7 +109,8 @@ osx_10.8_x86_64_dmg:
      - dmg
    variables:
      BUILD_TYPE_FLAG: -X
-     osx_version: 10.8
+   before_script:
+     - *fetch_nwjs
    script:
      - echo `pwd`
      - *build_all
@@ -114,6 +127,8 @@ windows_i386_innosetup:
     - i386
   variables:
     BUILD_TYPE_FLAG: -Z
+  before_script:
+     - *fetch_nwjs
   script:
     - echo `pwd`
     - *build_all
@@ -133,6 +148,7 @@ raspbian_stretch_armv7l_deb:
      BUILD_TYPE_FLAG: -r
    before_script:
      - *debian_image_preparation
+     - *fetch_nwjs
    script:
      - echo `pwd`
      - *build_all