feat: install most dependencies in cacheable layer, before adding main

Instead of requiring the whole stack to be reinstalled whenever a specific version (e.g. main) causes a cache fail, this aims to have stable default version with a second-stage installation of the potentially-modified versions of eicrecon and juggler. This should allow all builds to cache the first installation in virtually all cases, leading to faster builds.

.github/workflows/mirror.yaml

+name: Mirror and Trigger EICweb
+
+on:
+  pull_request:
+  push:
+  workflow_dispatch:
+
+concurrency:
+  group: mirror
+  cancel-in-progress: false
+
+jobs:
+  build:
+    name: Mirror and Trigger EICweb
+    runs-on: ubuntu-latest
+    permissions:
+      actions: write
+      contents: read
+    steps:
+    - name: Checkout
+      uses: actions/checkout@v4
+      with:
+        fetch-depth: 0
+    - name: Push to EICweb
+      uses: eic/gitlab-sync@master
+      with:
+        url: https://eicweb.phy.anl.gov/containers/eic_container.git/
+        token: ${{ secrets.GITLAB_TOKEN }}
+        username: ${{ secrets.GITLAB_USERNAME }}
+        ciskip: true
+    - name: Trigger EICweb
+      uses: eic/trigger-gitlab-ci@v2
+      with:
+        url: https://eicweb.phy.anl.gov
+        project_id: 290
+        token: ${{ secrets.EICWEB_EIC_CONTAINER_TRIGGER }}
+        ref_name: ${{ github.event.pull_request.head.ref || github.ref }}
+        variables:
+          GITHUB_REPOSITORY=${{ github.repository }}
+          GITHUB_SHA=${{ github.event.pull_request.head.sha || github.sha }}
+          GITHUB_PR=${{ github.event.pull_request.number }}