Fix usage of GitHub actions cache

[pziggo]

Using quotes for the items in path does not resolve ~ correctly and
thus leads to the following warning in the post section:

Warning: Path Validation Error: Path(s) specified in the action for
caching do(es) not exist, hence no cache is being saved.

[dastbe]

So it's actually because of how vertical bars vs. basic strings work.

testing these two files

test: "one"

vs

test: |
  "one"

here is the value of the the test key respectively:

# simple string
'one'

# pipe
'"one"\n'

Github will process these lines literally, hence why the quotes are confusing.

[kormide]

Thanks for the fix. Sorry that this slipped my radar.