use GITHUB_TOKEN for pages deployment

[milesfrain]

Suggested in #345

[milesfrain]

Don't know why we're getting a cache miss.

An earlier job creates the cache:

Cache not found for input keys: Linux-spago-969dc6fff69f8364c7c580e2da1a2c51397b149f53bacedb551a4d08ac5602f4
Cache saved with key: Linux-spago-969dc6fff69f8364c7c580e2da1a2c51397b149f53bacedb551a4d08ac5602f4

https://github.com/purescript-contrib/purescript-book/runs/2614884263?check_suite_focus=true

Then the job for this PR unexpectedly can't find that cache with the same key:

Cache not found for input keys: Linux-spago-969dc6fff69f8364c7c580e2da1a2c51397b149f53bacedb551a4d08ac5602f4

https://github.com/purescript-contrib/purescript-book/runs/2632204894?check_suite_focus=true

[milesfrain]

Don't know why we're getting a cache miss.

Looks like this is due to security settings where caches are only available to branch descendants. One fix is to create these caches on the default / master branch too.
https://docs.github.com/en/actions/guides/caching-dependencies-to-speed-up-workflows#restrictions-for-accessing-a-cache