Add optional Nix CI

As the whole Nix flake in this repo is experimental, this workflow
has `continue-on-error` set to true in order to avoid potential problems
with Nix stalling CCM development. We can make it required, or even
completely switch to it, after some time, to be sure Nix really works
here.

.github/workflows/nix.yml

@@ -0,0 +1,58 @@
+name: CCM Integration tests (using Nix)
+
+on:
+  push:
+    branches:
+     - master
+     - next*
+
+  pull_request:
+    branches:
+     - next*
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    continue-on-error: true
+
+    steps:
+    - uses: actions/checkout@v3
+    - uses: cachix/install-nix-action@v20
+      with:
+        github_access_token: ${{ secrets.GITHUB_TOKEN }}
+
+    - name: Cache binary versions
+      id: cache-versions
+      uses: actions/cache@v2
+      with:
+        path: |
+          ~/.ccm/repository
+          ~/.ccm/scylla-repository
+        key: ${{ runner.os }}-binaries
+
+    - name: Download versions
+      if: steps.cache-versions.outputs.cache-hit != 'true'
+      run: |
+        if [ ! -f ~/.ccm/scylla-repository/unstable/master/2023-04-13T13_31_00Z ]; then
+          RELOC_VERSION="2023-04-13T13:31:00Z"
+        
+          nix develop --command ./ccm create temp -n 1 --scylla --version unstable/master:${RELOC_VERSION}
+          nix develop --command ./ccm remove
+        fi
+        nix develop --command ./ccm create temp-cas -n 1 --version 3.11.4 > /dev/null
+        nix develop --command ./ccm remove
+
+    - name: Test with pytest (python 3.9)
+      run: |
+        nix develop --command python3.9 -m pytest ./tests -x
+
+    - name: Test with pytest (python 3.11)
+      run: |
+        nix develop --command python3.11 -m pytest ./tests -x
+
+    - name: Copy logs/results
+      if: contains(github.event.pull_request.labels.*.name, 'PR-upload-log')
+      uses: actions/upload-artifact@v2
+      with:
+        name: ccm-tests-log
+        path: tests/test_results/