[CI] Reorganise GitHub Actions setup

* move directory to the right name
* remove reference to `master` branch
* add `paths` triggers
* add concurrency groups
* update `actions/checkout` workflow to latest version

Author: giordano

.github/actions/CI.yml

@@ -0,0 +1,59 @@
+name: Run tests
+
+on:
+  push:
+    branches:
+      - main
+    paths:
+      - '.github/workflows/UnitTests.yml'
+      - 'ext/**'
+      - 'src/**'
+      - 'test/**'
+      - 'Project.toml'
+  pull_request:
+    paths:
+      - '.github/workflows/UnitTests.yml'
+      - 'ext/**'
+      - 'src/**'
+      - 'test/**'
+      - 'Project.toml'
+
+# needed to allow julia-actions/cache to delete old caches that it has created
+permissions:
+  actions: write
+  contents: read
+
+concurrency:
+  # Skip intermediate builds: always.
+  # Cancel intermediate builds: always.
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  test:
+    runs-on: ${{ matrix.os }}
+    strategy:
+      matrix:
+        julia-version:
+          - 'min'
+          - '1'
+          - 'pre'
+        julia-arch:
+          - x64
+          - x86
+        os:
+          - ubuntu-latest
+          - windows-latest
+          - macOS-latest
+        exclude:
+          - os: macOS-latest
+            julia-arch: x86
+
+    steps:
+      - uses: actions/checkout@v5
+      - uses: julia-actions/setup-julia@v2
+        with:
+          version: ${{ matrix.julia-version }}
+          arch: ${{ matrix.julia-arch }}
+      - uses: julia-actions/cache@v2
+      - uses: julia-actions/julia-runtest@v1