Update GitHub workflows to pinned Stack version

These were out of sync with the intended version (specified in
.vscode/settings.json).

Author: sellout

.github/workflows/bundle-ucm.yaml

@@ -40,7 +40,7 @@ jobs:
       - name: install stack
         uses: unisonweb/actions/stack/install@main
         with:
-          stack-version: 2.15.5
+          stack-version: 2.15.7
 
       - name: build
         run: |

.github/workflows/ci.yaml

@@ -110,7 +110,7 @@ jobs:
         if: steps.cache-ucm-binaries.outputs.cache-hit != 'true'
         uses: unisonweb/actions/stack/install@main
         with:
-          stack-version: 2.15.5
+          stack-version: 2.15.7
 
       # 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

@@ -28,7 +28,7 @@ jobs:
       - name: install stack
         uses: unisonweb/actions/stack/install@main
         with:
-          stack-version: 2.15.5
+          stack-version: 2.15.7
 
       - name: build with haddocks
         working-directory: unison

.github/workflows/update-transcripts.yaml

@@ -23,7 +23,7 @@ jobs:
       - name: install stack
         uses: unisonweb/actions/stack/install@main
         with:
-          stack-version: 2.15.5
+          stack-version: 2.15.7
 
       # One of the transcripts fails if the user's git name hasn't been set.
       - name: set git user info