fix(ci): read pnpm version from devEngines in healthchat desktop webapp job

pnpm/action-setup@v4 errored 'No pnpm version specified' because this repo
declares pnpm via devEngines.packageManager, not the top-level packageManager
field. Bump to v6.0.8 (as the shared desktop reusables use), which reads it.

.github/workflows/build-healthchat-desktop.yaml

@@ -99,7 +99,9 @@ jobs:
                   ref: ${{ steps.workspace.outputs.ref }}
                   persist-credentials: false
 
-            - uses: pnpm/action-setup@v4
+            # v6.0.8 reads the pnpm version from devEngines.packageManager in package.json
+            # (this repo has no top-level "packageManager" field, which older majors require).
+            - uses: pnpm/action-setup@0e279bb959325dab635dd2c09392533439d90093 # v6.0.8
             - name: Setup Node.js
               uses: actions/setup-node@v4
               with: