Use custom workspace for running action in

There's been a few issues with Jekyll-Diff-Action
using the default `/github/workspace` directory
to run in. That's mounted in read only mode, so
any command that tried modify it failed (see PR #1
and issue #2).
To get away from all this, this PR adds an initial
step to the diff script to create our own workspace
with the appropriate permissions.

jekyll_diff.sh

@@ -6,6 +6,19 @@ set -e # stop executing after error
 echo "Starting Jekyll diff script"
 
 
+####################################################
+# Create workspace for Jekyll Diff Action
+####################################################
+
+mkdir /jekyll-diff-action
+mkdir /jekyll-diff-action/new
+mkdir /jekyll-diff-action/old
+mkdir /jekyll-diff-action/workspace
+cp --archive /github/workspace/. /jekyll-diff-action/workspace
+chmod -R a+w /jekyll-diff-action
+cd /jekyll-diff-action/workspace
+
+
 ####################################################
 # Determine whether it's a PR or Commit
 ####################################################
@@ -25,13 +38,10 @@ fi
 # Build both site versions and determine diff
 ####################################################
 
-cd /github/workspace
-mkdir -p .jekyll-cache # workaround for https://github.com/jekyll/jekyll/issues/7591
-
-jekyll build --trace --destination /tmp/new/
+jekyll build --trace --destination /jekyll-diff-action/new/
 git checkout $common_ancestor_sha
-jekyll build --trace --destination /tmp/old
-diff="$(diff --recursive --new-file --unified=0 /tmp/old /tmp/new || true)" # 'or true' because a non-identical diff outputs 1 as the exit status
+jekyll build --trace --destination /jekyll-diff-action/old
+diff="$(diff --recursive --new-file --unified=0 /jekyll-diff-action/old /jekyll-diff-action/new || true)" # 'or true' because a non-identical diff outputs 1 as the exit status
 
 
 ####################################################