Bump stack version in CI

.github/workflows/bundle-ucm.yaml

@@ -39,6 +39,8 @@ jobs:
 
       - name: install stack
         uses: unisonweb/actions/stack/install@main
+        with:
+          stack-version: 2.15.5
 
       - name: build
         run: |

.github/workflows/ci.yaml

@@ -102,6 +102,8 @@ jobs:
       - name: install stack
         if: steps.cache-ucm-binaries.outputs.cache-hit != 'true'
         uses: unisonweb/actions/stack/install@main
+        with:
+          stack-version: 2.15.5
 
       # Build deps, then build local code. Splitting it into two steps just allows us to see how much time each step
       # takes.

.github/workflows/haddocks.yaml

@@ -27,6 +27,8 @@ jobs:
 
       - name: install stack
         uses: unisonweb/actions/stack/install@main
+        with:
+          stack-version: 2.15.5
 
       - name: build with haddocks
         working-directory: unison

.github/workflows/update-transcripts.yaml

@@ -22,6 +22,8 @@ jobs:
 
       - name: install stack
         uses: unisonweb/actions/stack/install@main
+        with:
+          stack-version: 2.15.5
 
       # One of the transcripts fails if the user's git name hasn't been set.
       - name: set git user info