CI: avoid mixing caches from different OSes

This is a bug in setup-ocaml@v3 ocaml/setup-ocaml#839

Work it around by defining our own cache prefix based on runner OS version.

Unfortunately the version itself doesn't seem to be available as a variable in GH actions.
There is 'runner.os', but that is just a generic Linux, there is 'matrix.os', but that is only present when using a matrix,
and there is '..container' which is only present when containers are used.

Use another GH action to determine the version, and now the cache-prefix looks like this:
```
cache-prefix: v3-Ubuntu-22.04
```

Signed-off-by: Edwin Török <edwin.torok@cloud.com>

Author: edwintorok

.github/workflows/setup-xapi-environment/action.yml

@@ -12,6 +12,7 @@ runs:
       shell: bash
       run: |
         curl --fail --silent https://raw.githubusercontent.com/xapi-project/xs-opam/master/tools/xs-opam-ci.env | cut -f2 -d " " > .env
+        cat /etc/os-release
 
     - name: Download XE_SR_ERRORCODES.xml
       shell: bash
@@ -42,6 +43,10 @@ runs:
         echo "TMPDIR=${TMPDIR}" >>"$GITHUB_ENV"
         echo "XDG_CACHE_HOME=${XDG_CACHE_HOME}" >>"$GITHUB_ENV"
 
+    - name: Get runner OS info
+      uses: kenchan0130/actions-system-info@master
+      id: system-info
+
     # We set DUNE_CACHE_STORAGE_MODE, it is required for dune cache to work inside opam for now,
     # otherwise it gets EXDEV and considers it a cache miss
     - name: Use ocaml
@@ -52,6 +57,7 @@ runs:
           xs-opam: ${{ steps.dotenv.outputs.repository }}
         dune-cache: true
         opam-pin: false
+        cache-prefix: v3-${{ steps.system-info.outputs.name }}-${{ steps.system-info.outputs.release }}
       env:
         DUNE_CACHE_STORAGE_MODE: copy