ci: use bun ci for dependency installation in ci

Update the dependency installation command from 'bun install' to 'bun ci'
in both the test and nix-flake-test jobs. This ensures reproducible builds
by installing from the lockfile without updating it.

Author: MBanucu

.github/workflows/ci.yml

@@ -36,7 +36,7 @@ jobs:
             ${{ runner.os }}-bun-
 
       - name: Install dependencies
-        run: bun install
+        run: bun ci
 
       - name: Install Playwright Browsers
         run: bunx playwright install --with-deps
@@ -82,7 +82,7 @@ jobs:
           restore-keys: |
             ${{ runner.os }}-bun-
       - name: Install dependencies (Nix devShell)
-        run: nix develop .# --command bun install
+        run: nix develop .# --command bun ci
 
       - name: Build (Nix devShell)
         run: nix develop .# --command bun build:prod