ci: Workaround for stack/pantry caching of github archives

Turns out `commit: master` doesn't really mean latest master, because
it's cached. It sometimes works, because when the GHA cache comes from
a job that depended on xmonad-0.15, then the master archive isn't cached
and is fetched. When the cache comes from a job that previously fetched
master, it's not fetched again.

The least ugly way to fix it I found is to use the `[<refname>]@{<date>}`
git revision format, and substitute current date/time before invoking
stack. To make stack-master.yaml valid without the substitution, we use
`master@{today}` which is a valid git revision that GitHub understands.

.github/workflows/stack.yml

@@ -96,6 +96,10 @@ jobs:
       - name: Build and test
         run: |
           set -ex
+
+          # workaround for stack/pantry caching of github archives
+          sed -e "s/@{today}/@{$(date -u --iso-8601=seconds)}/" -i ${{ matrix.yaml }}
+
           stack test \
             --fast --no-terminal \
             --stack-yaml=${{ matrix.yaml }} \

stack-master.yaml

@@ -8,4 +8,4 @@ packages:
 
 extra-deps:
 - github: xmonad/xmonad
-  commit: master
+  commit: master@{today}