remove duplicate CI actions for PR+branch, speed up pixi install, fix install bug in ci

Author: arijit-dasgupta

.github/workflows/ci.yml

@@ -9,6 +9,12 @@ on:
       - main
       - master
 
+# Prevent duplicate runs when pushing to a branch with an open PR
+# Groups PRs and their corresponding branch pushes together
+concurrency:
+  group: ${{ github.workflow }}-${{ github.event.pull_request.head.ref || github.ref_name }}
+  cancel-in-progress: true
+
 jobs:
   format-and-lint:
     name: Format and Lint Check
@@ -22,9 +28,21 @@ jobs:
         uses: prefix-dev/[email protected]
         with:
           pixi-version: "latest"
+          cache: ${{ github.event_name == 'pull_request' }}  # Only cache on PRs
+      
+      - name: Cache pixi environment
+        if: github.event_name == 'pull_request'
+        uses: actions/cache@v4
+        with:
+          path: |
+            ~/.pixi
+            .pixi
+          key: ${{ runner.os }}-pixi-${{ hashFiles('pixi.lock') }}
+          restore-keys: |
+            ${{ runner.os }}-pixi-
       
       - name: Install dependencies
-        run: pixi install --all-environments
+        run: pixi install
 
       - name: Check formatting
         run: pixi run format-check