ci/d: Use version instead of hash

There are plenty of other actions in this repository using a version
that is not certified by Github. The disclaimer makes this obvious.
The action in question is not simply a user's action, but maintained
by dlang-community, which is overseen by the D Language Foundation.
The recent set of changes to actions means that this workflow is currently
broken, as it was previously using add-path and set-env.
Keeping on upgrading the hash will provide a terrible user experience,
as it is likely to be forgotten, and will also require user-intervention
when another set of change breaks this action, hence why this updates
the hash to a version instead of changing it to the latest hash.

ci/d.yml

@@ -17,7 +17,7 @@ jobs:
 
     steps:
     - uses: actions/checkout@v2
-    - uses: dlang-community/setup-dlang@7c3e57bdc1ff2d8994f00e61b3ef400e67d2d7ac
+    - uses: dlang-community/setup-dlang@v1
 
     - name: 'Build & Test'
       run: |